Master Cylinder Rebuild - DIY (1 Viewer)
- Thread starter Skidoo
- Start date
This site may earn a commission from merchant affiliate
links, including eBay, Amazon, Skimlinks, and others.
Simple green didn't end up cleaning much of it at all. It comes off very easily with a simple wipe, but because the areas are inaccessible I just bought a new reservoir tank from the local toyota shop.
whatever the residue is/was likely sloshed up inside the reservoir while driving and stuck to the reservoir.
I think I'm going to go ahead and do the master cylinder rebuild while I'm at it, wondering if perhaps the squealing sound is coming from something leaking past a seal when the motor is trying to build up pressure.
whatever the residue is/was likely sloshed up inside the reservoir while driving and stuck to the reservoir.
I think I'm going to go ahead and do the master cylinder rebuild while I'm at it, wondering if perhaps the squealing sound is coming from something leaking past a seal when the motor is trying to build up pressure.
After replacing the master cylinder reservoir and the plunger replacement I spent 30-40 mins bleeding the brakes with someone sitting pumping the pedal, and the ABS bleed procedure with tech stream, pump ran great, and had no screeching sounds. After doing the 40 brake pedal presses, the pump would run for 30ish seconds and then stop
For fun I went out this AM when the temps were a little lower (low 60s, vs high 80s while bleeding) and put the key in the "on" position to let the pump run. It ran for 3+ mins before I turned the key to the off-position. The screeching had returned. But the pump ran as expected (without screeching sounds) for 30-40 seconds)
I have felt around when the noise is happening and can feel the red high-pressure line from the pump vibrate when the noise happens.
the pump works, I can drain the accumulator (40 presses on the brake pedal) and pretty quickly the level in the reservoir will go down as the accumulator builds pressure.
I'm wondering if there is a good way to test the high pressure sensor. The pump seems to be working, and I'm wondering if the pressure sensor is malfunctioning and causing and over-pressure situation.
Another thought is that the pump has some worn seal, and that is causing fluid to make it past a seal as it's trying to build pressure.
My experience over the last few weeks is that the issue happens first thing in the day, but after the pump successfully builds pressure, it works find for the remainder of the day. Doesn't seem to matter if the car is shut off for a few hours, started back up (rinse and repeat 2-3 times) or if we are cruising down the highway with regular slow-downs and stops for gas/food/restroom. The issue only happens first thing in the AM, and not after everything is warmed up.
I'm not sure if this points to a bad seal inside the pump or the brake booster system, or a sensor that is faulty and only starts to work once it is warmed up.
I'll plug tech-stream back in today and see what errors exist (if any) with the brake system, that might help point in a certain direction.
For fun I went out this AM when the temps were a little lower (low 60s, vs high 80s while bleeding) and put the key in the "on" position to let the pump run. It ran for 3+ mins before I turned the key to the off-position. The screeching had returned. But the pump ran as expected (without screeching sounds) for 30-40 seconds)
I have felt around when the noise is happening and can feel the red high-pressure line from the pump vibrate when the noise happens.
the pump works, I can drain the accumulator (40 presses on the brake pedal) and pretty quickly the level in the reservoir will go down as the accumulator builds pressure.
I'm wondering if there is a good way to test the high pressure sensor. The pump seems to be working, and I'm wondering if the pressure sensor is malfunctioning and causing and over-pressure situation.
Another thought is that the pump has some worn seal, and that is causing fluid to make it past a seal as it's trying to build pressure.
My experience over the last few weeks is that the issue happens first thing in the day, but after the pump successfully builds pressure, it works find for the remainder of the day. Doesn't seem to matter if the car is shut off for a few hours, started back up (rinse and repeat 2-3 times) or if we are cruising down the highway with regular slow-downs and stops for gas/food/restroom. The issue only happens first thing in the AM, and not after everything is warmed up.
I'm not sure if this points to a bad seal inside the pump or the brake booster system, or a sensor that is faulty and only starts to work once it is warmed up.
I'll plug tech-stream back in today and see what errors exist (if any) with the brake system, that might help point in a certain direction.
uHu
Bridgeburner
Could it that some boosters develop a noisy Check Valve, and others don't? E.g. because of a period of high water content in the fluid.
Could the type of brake fluid affect the noise? I use OEM Dot 5.1, as recommended by my dealer (partly owned by Toyota Motor Europe) and don't have this kind of noise.
I'm not thinking check valve at this point. If the check valve were to fail, when the pump turned off, the fluid would rush back into the reservoir when the pump stopped. I only get this issue when the pump is running, and the check valve should be open (although if the check valve was closed, that could cause an issue, but I'm pretty sure it's opening)
you can see where the check valve is located from this link: http://www.ezdries.net/Vidpics/ATRAC/LC100 new features.pdf
Although, the system does lose pressure over-night, so the pressure is escaping somewhere. However, I think this is normal behavior.
BUT WAIT, now that I think about it.... when the screech happens, I do get a bubble of fluid back into the reservoir tank... I don't think it's check valve related, but might be related to a fully pressurized tank, and fluid going backwards through the pump and bubbling back into the reservoir.
I'm thinking high pressure switch that doesn't correctly operate until it is a bit warmer. Similar to this post: Abs issue some work done but stuck?
uHu
Bridgeburner
I suppose the check valve is just a common spring loaded ball valve, seated i a carefully machined recess for sealing. It opens only because of the pump pressure overcoming the spring tension.
Re holding pressure: My 2000 held the pressure overnight for 9 years, running only <5 sec every morning, while my 2004 had already in 2009 (when I got it) started to loose the pressure within 30-60 mins. It's still working well tho'.
Re holding pressure: My 2000 held the pressure overnight for 9 years, running only <5 sec every morning, while my 2004 had already in 2009 (when I got it) started to loose the pressure within 30-60 mins. It's still working well tho'.
2001LC
SILVER Star
When cold OAT, rubber seals shrink. Very common in colorado winters, to have brake warning sound, on cold mornings. Then as we use brakes, seal(s) heat and expand, alarm turns off. If fluid has been in too long, it can damage seals and pit piston. We put on new pads and no more warning. Than as pads wear, worst of pitting on piston get exposed to the seals and pressure warning again (cold morning alam)
Not flushing brake fluid regularly is damaging to all seals. This includes master. If your fluid turns dark quickly after flushing, this is a bad sign of seal damage.
If squeal sometimes referred to a screaming banshee. Is also accompanied by air bubbles in reservoir. Only thing that can be done is replace master. There is a seal we can't get to. This "seals damage" allows pressurized brake fluid past it. That's what we hear. The bubbles in reservoir is the fluid released into from seal under pressure.
What I can not say, is which fluid is best. Toyota fluid has seal modifiers others may not. Toyota fluid in the can, is not same mix as what comes in the rig from factory. But is or was the same manufacture making it.
Your brakes will still work, provided squeal from bad seal in master or leaks at piston calliper seals, not so bad you lose most all pressure or fluid.. But, the constant excessive run time of pump, do to pressure lose. Will cause pump to wear out prematurely. Than we lose brakes, as we no pump to bring up pressure.
The HP switch can be tested along with pressure of system. But good luck finding a gauge to do the job. I have replaced the HP switch seal(s). I did get some improvement. But this seal we can't get to. That squealing. Is deep inside master cylinder behind plunger, which is the one we can not replace.
Note: The HP switch was first used in the 2000 USA models. In mid 2000 they change the switch. They are not interchangeable, the seal(s) are. But FSM states in the second gen switch, they are not reusable.
Not flushing brake fluid regularly is damaging to all seals. This includes master. If your fluid turns dark quickly after flushing, this is a bad sign of seal damage.
If squeal sometimes referred to a screaming banshee. Is also accompanied by air bubbles in reservoir. Only thing that can be done is replace master. There is a seal we can't get to. This "seals damage" allows pressurized brake fluid past it. That's what we hear. The bubbles in reservoir is the fluid released into from seal under pressure.
What I can not say, is which fluid is best. Toyota fluid has seal modifiers others may not. Toyota fluid in the can, is not same mix as what comes in the rig from factory. But is or was the same manufacture making it.
Your brakes will still work, provided squeal from bad seal in master or leaks at piston calliper seals, not so bad you lose most all pressure or fluid.. But, the constant excessive run time of pump, do to pressure lose. Will cause pump to wear out prematurely. Than we lose brakes, as we no pump to bring up pressure.
The HP switch can be tested along with pressure of system. But good luck finding a gauge to do the job. I have replaced the HP switch seal(s). I did get some improvement. But this seal we can't get to. That squealing. Is deep inside master cylinder behind plunger, which is the one we can not replace.
Note: The HP switch was first used in the 2000 USA models. In mid 2000 they change the switch. They are not interchangeable, the seal(s) are. But FSM states in the second gen switch, they are not reusable.
Last edited:
I went out today after things have warmed up (low 90s vs low 60s). I drained the pressure from the accumulator earlier this morning, and the pump ran for about 30 seconds and then stopped (it actually ran for 5-10 seconds, and then stopped, and then ran for another 20 seconds).
The pump didn't make the screeching sound at all during this 30 seconds.
All of this is leading me toward an issue related to some kind of sensor. The pump works, it can and does build pressure. I need to figure out how to measure the sensor itself to know whether or not it is properly functioning.
The pump didn't make the screeching sound at all during this 30 seconds.
All of this is leading me toward an issue related to some kind of sensor. The pump works, it can and does build pressure. I need to figure out how to measure the sensor itself to know whether or not it is properly functioning.
This guy suggests that the spring loaded valve that is part of the accumulator might be the problem.
The video is from an FJ, but those guys run into the same brake booster problems.
It seems unlikely to me that this would be the issue, because when the pump is running, the check valve should be solidly open, unless there is SO much pressure built up in the accumulator that it is able to push back against the pump and force the valve to close or rapidly open and close.
I did monitor several of the data readouts today while after things have warmed up (and when the pump seems to work just fine). What I found is that the master cylinder pressure sensor is really just a measure of how hard you are pressing the brake pedal. The firmer you press the brake pedal, the nearer to 4.0 volts the sensor reads. I can't imagine that causing what I think is an over-pressure situation within the booster. Unless it is registering that I'm slamming on the brake pedal when I'm not.
That leaves the two switches, the low pressure and high pressure... and those appear to be internal to the booster.
The video is from an FJ, but those guys run into the same brake booster problems.
It seems unlikely to me that this would be the issue, because when the pump is running, the check valve should be solidly open, unless there is SO much pressure built up in the accumulator that it is able to push back against the pump and force the valve to close or rapidly open and close.
I did monitor several of the data readouts today while after things have warmed up (and when the pump seems to work just fine). What I found is that the master cylinder pressure sensor is really just a measure of how hard you are pressing the brake pedal. The firmer you press the brake pedal, the nearer to 4.0 volts the sensor reads. I can't imagine that causing what I think is an over-pressure situation within the booster. Unless it is registering that I'm slamming on the brake pedal when I'm not.
That leaves the two switches, the low pressure and high pressure... and those appear to be internal to the booster.
2001LC
SILVER Star
Whereas this may have help that guys FJ, or he thought it did at time of video. Possible it was not have been the same issue (screaming banshee with air bubbles).
More than one person in mud has replaced the silencer pipe and spring. Or replaced the entry booster assembly (wire, motor, pump & accumulator) along with master cylinder plunger & seals The screaming banshee with air bubbles returned.
More than one person in mud has replaced the silencer pipe and spring. Or replaced the entry booster assembly (wire, motor, pump & accumulator) along with master cylinder plunger & seals The screaming banshee with air bubbles returned.
True true.... I guess I'm inclined to throw a hundred bucks at it to see if I can get something to stick vs dropping a couple thousand at it. I'm not sure I'd go with a reman unit, and would probably buy a new unit. I'd rather not have to deal with this ever again.
Last night I depressurized the system (40 pumps on the brake pedal). Today, similar environmental temps to yesterday (when I last had screeching). Pump ran for the ~30 seconds it was supposed to and turned off. This makes me think that it might have more to do with the residual pressure bleeding off over-night and not the ambient temperature.
The only thing I did that was a little different today was pulled the relays so that the motor wouldn't run so I could get techstream up and running and into the right screen to monitor data points. That could have allowed something to get a little power and warm up.
My plan now is to let it sit for the day to let the residual pressure bleed off, go back tonight and test again.
I'm also thinking I might bypass the motor relay and kick the pump on to see if I can recreate the sound. That could help confirm that the sound comes from over pressure somewhere in the system.
2001LC
SILVER Star
Basically I did the same. Client let me work my way through his issues. It was a big experiment.
I knew part of his problem was caliper pistons & seal. The brake bleeders were missing bleeders cap and frozen bleeders in the rear calipers. He paid 3 shops over 5 year to bleed the brakes. It turns out not one did the job. Slee, noted they did not flush, rear bleeders frozen in, 5 years earlier (he did not read the work order after pick up). Other shops charged him, but did not flush/bleed.
I took master out 3 different times, over period of months through the winter. I was testing in OATs down to 13 F. At first condition of long booster run time when cold, started at 32F. Brake alarms were coming on also. Thy DTCs, were dealing with pressure and long run times of booster motor. I got work normal down to 13F overnight (coldest we saw). I really though I had it solved with HP switch seals. But than client reported screaming banshee with air bubbles returned. So we were down to the one seal in thee master cylinder and the ABS unit HP & LP switches. Bubbles coming up, points to the master seal, that is not replaceable.
In the end the master took care of screaming banshee with air bubbles.
Now he's a total new system end to end even flexible line. The only the hard lines were not replaced.
Long booster run time no screaming banshee with air bubbles:
To just have long booster motor run times, is not the same as screaming banshee with air bubbles. Long runs time can be air in system and or leaks, among other less frequent issues. Caliper piston seal being one place weep is very hard to detect. Bleeders weeping is very common after bleeding/flushing, which yields long run times (44 sec 1 bleeder weep, 48 sec 2 bleeder weep).
I left the system pressurized over-night. Based on the tape I left on the side of the reservoir, very little pressure was lost (the level went up maybe 1/8th of an inch)
Went out this morning, and the lower volume screeching started almost right away. I let the pump run for a bit while I was waiting for techstream to start up. The pump did stop running before the loud screeching started.
I did the 40+ pump on the brake pedal, the pump them quickly ran again for the short 30 seconds that it should have run in. I'm thinking that the solution might be to depressurize the system at the end of the day.
I'm wondering if perhaps sitting pressurized overnight is causing the high pressure switch to get stuck on.
I'm going to have to figure out how to measure the status of the switch without having to turn the ignition on.
Went out this morning, and the lower volume screeching started almost right away. I let the pump run for a bit while I was waiting for techstream to start up. The pump did stop running before the loud screeching started.
I did the 40+ pump on the brake pedal, the pump them quickly ran again for the short 30 seconds that it should have run in. I'm thinking that the solution might be to depressurize the system at the end of the day.
I'm wondering if perhaps sitting pressurized overnight is causing the high pressure switch to get stuck on.
I'm going to have to figure out how to measure the status of the switch without having to turn the ignition on.
Recently had spongy mushy brakes driving to my neighbor's house (thank god it was just around the block) - and got to reading all the 100 series failure issues. After checking all of the lines and the area around the MC under the hood and by the brake pedal for cracks and leaks - and not finding any, I ordered the new MC piston. THEN, I realized that I had overfilled my reservoir about 3 weeks ago, since I hadn't followed the procedure of pumping the pedal to evacuate the accumulator first. OOPS, newbie owner of a 100 series - lots of electronics to be aware of - my 60 is so much less complicated.
This morning I removed the excess brake fluid using a 300ML syringe and hose, then ran the pump motor run-time test (30-40 sec is spec). Test #1 - 40 sec, Test #2 30 sec. Good enough.
Satisfied with the results I took it for a spin and had my brakes back to normal: little bit of play but great stopping power and confirmed the ABS was working to slow me down from 40MPH hard braking runs.
At this point I'm going to leave the system alone and read the manual before changing anything else.
Thanks to all the Land Cruiser Sleuths who have helped make owning and maintaining these cool rigs possible.
PS- Also check the resistance wire leads under the ABS module - shiny and beautiful. I added a bit of dielectric grease just to be on the safe side.
This morning I removed the excess brake fluid using a 300ML syringe and hose, then ran the pump motor run-time test (30-40 sec is spec). Test #1 - 40 sec, Test #2 30 sec. Good enough.
Satisfied with the results I took it for a spin and had my brakes back to normal: little bit of play but great stopping power and confirmed the ABS was working to slow me down from 40MPH hard braking runs.
At this point I'm going to leave the system alone and read the manual before changing anything else.
Thanks to all the Land Cruiser Sleuths who have helped make owning and maintaining these cool rigs possible.
PS- Also check the resistance wire leads under the ABS module - shiny and beautiful. I added a bit of dielectric grease just to be on the safe side.
Hey guys. So I have a 2000 LC with a screaming something in the MC. I also have a crashed 2003 4Runner. I pulled the master from the 4 runner and man are they close. The only difference I see is a plug,reservoir shape and the mounting stud locations. So I thought the screech was the pump motor and accumulator. I swapped them as the are the same and I thought I had it fixed. Well no. Same screech came on 2 blocks after I finished. I didn’t do a lot of bleeding. But figured I’d Post that it seems to be something else. I am considering swapping the plug and drilling the firewall to mount this MC. Also removing the shaft extension for the pedal to make it the correct length. Am I crazy?
@silveradofab great update, thanks for sharing!
This basically confirms (in my mind) that the check-valve in the pump assembly isn't at fault (at least not in yours). For me, this also confirms that the high pressure switch is getting stuck on and not properly detecting when the pressure is high enough. This causes the pump to attempt to continue to pressurize, and I'm guessing because no more pressure can be built up, some type of cavitation is happening and causing the screeching sound, as well as causing bubbles to go back up into the reservoir.
I made a mark on my reservoir where the pump normally stops (when it stops). When I get the sound, I can visually see the brake fluid go below that line and the sound then starts. I don't get the sound before the fluid is below that line (ie, not enough pressure is built up, and the accumulator can still take a little more pressure)
cutting new holes for the 4runner version would probably work. One thing you might want to try is to hook everything up as best you can, electronics, and brake lines to the 4runner assembly in your 100. It might not "fit" enough to drive, but if everything is hooked up, you can turn the 100 on and see if it chokes due to a slightly different expectation when it comes to what is plugged into the electronics.
By retrofitting the 4runner part in, you're setting yourself up for some challenges down the road for future service should it be needed, because now your kind of troubleshooting a 100 and a 4runner. Also, what if the 4runner assembly is only tuned to deliver X amount of braking force, but the 100 needs X+Y amount of braking force.
I think the next step for me is to locate a donor pump assembly (or experiment on mine) taking it apart to look for the mysterious high pressure switch to see if a part number can be located. Hydraulic systems are not new and are not unique to the 100 series. I suspect these are "common" parts, if you know where to go and look.
This basically confirms (in my mind) that the check-valve in the pump assembly isn't at fault (at least not in yours). For me, this also confirms that the high pressure switch is getting stuck on and not properly detecting when the pressure is high enough. This causes the pump to attempt to continue to pressurize, and I'm guessing because no more pressure can be built up, some type of cavitation is happening and causing the screeching sound, as well as causing bubbles to go back up into the reservoir.
I made a mark on my reservoir where the pump normally stops (when it stops). When I get the sound, I can visually see the brake fluid go below that line and the sound then starts. I don't get the sound before the fluid is below that line (ie, not enough pressure is built up, and the accumulator can still take a little more pressure)
cutting new holes for the 4runner version would probably work. One thing you might want to try is to hook everything up as best you can, electronics, and brake lines to the 4runner assembly in your 100. It might not "fit" enough to drive, but if everything is hooked up, you can turn the 100 on and see if it chokes due to a slightly different expectation when it comes to what is plugged into the electronics.
By retrofitting the 4runner part in, you're setting yourself up for some challenges down the road for future service should it be needed, because now your kind of troubleshooting a 100 and a 4runner. Also, what if the 4runner assembly is only tuned to deliver X amount of braking force, but the 100 needs X+Y amount of braking force.
I think the next step for me is to locate a donor pump assembly (or experiment on mine) taking it apart to look for the mysterious high pressure switch to see if a part number can be located. Hydraulic systems are not new and are not unique to the 100 series. I suspect these are "common" parts, if you know where to go and look.
I was curious about pressures. I do all my own (or a close friend) wrenching. That part doesn’t bother me. Neither does drilling the fire wall. It is soooo similar I doubt it’s different. Has anyone opened it up far
Enough to locate the high pressure switch you are taking about? I have read nearly this entire thread and can’t remember.
Enough to locate the high pressure switch you are taking about? I have read nearly this entire thread and can’t remember.
@silveradofab I hear you... with my '03 approaching 18 years old, I'm more comfortable with drilling new holes in things to keep it on the road. I would be grateful if you tore into your screeching pump electronics to look for something that indicates the high pressure switch is broken.
I did see pictures of 4runner units and the plunger on those is longer, but it looks like something is just screwed onto the end of it, and would be a quick swap. You would also need to swap the pressure sensor in the front of the unit, that sensor feeds into the computer to indicate how much pressure is being applied to the brake pedal.
I posted a simple diagram I found, that would probably help trace where the valves are. I think someone pulled apart something that shows the tops of the solenoids (see image below).
Looking back at the diagram there is a relief valve just after the high & low switches that leads back to the tank. I was thinking that the sound was some kind of over-pressurization scenario and possible cavitation, but with it could be (and likely is) the relief valve flapping open and closed quickly. It must take serious pressure to open it, because it needs to normally stay closed when the system is fully pressurized!
Also the tube locations going back to the reservoir in the diagram are not in the correct locations, the middle tube leads to the pump. So the relief valve could very well be the front tube of the reservoir
I'm guessing that the switches are on the left side of that photo (teal colored).
It looks like there are a few screws that can be undone to further disassemble.
It also looks like this cover could be removed while the unit is in the vehicle, this would make it VERY easy to read voltages/resistances those teal components (if they are the switches).
I'm hopeful that the electronic components are threaded in place and are not permanently bonded.
I did see pictures of 4runner units and the plunger on those is longer, but it looks like something is just screwed onto the end of it, and would be a quick swap. You would also need to swap the pressure sensor in the front of the unit, that sensor feeds into the computer to indicate how much pressure is being applied to the brake pedal.
I posted a simple diagram I found, that would probably help trace where the valves are. I think someone pulled apart something that shows the tops of the solenoids (see image below).
Looking back at the diagram there is a relief valve just after the high & low switches that leads back to the tank. I was thinking that the sound was some kind of over-pressurization scenario and possible cavitation, but with it could be (and likely is) the relief valve flapping open and closed quickly. It must take serious pressure to open it, because it needs to normally stay closed when the system is fully pressurized!
Also the tube locations going back to the reservoir in the diagram are not in the correct locations, the middle tube leads to the pump. So the relief valve could very well be the front tube of the reservoir
I'm guessing that the switches are on the left side of that photo (teal colored).
It looks like there are a few screws that can be undone to further disassemble.
It also looks like this cover could be removed while the unit is in the vehicle, this would make it VERY easy to read voltages/resistances those teal components (if they are the switches).
I'm hopeful that the electronic components are threaded in place and are not permanently bonded.
2001LC
SILVER Star
I really do not think you can do anything with the switch(s) in the ABS unit (black plastic box on side of master). If you do, it could created unsafe braking. You can find these ABS units from a parts rigs. Try one and see what happens. If that is the issue it would be and easy fix. I'd be very happy to see this work!
But if HP or LP within ABS unit switch bad, I would think you get a DTC & dash lights. Which I have seen and replaced the ABS unit to correct. But no screeching or bubbles that I knew of or reported to me, in that one.
I'm now of the opinion. That when we get the screech with air bubbles seen in reservoir. We must replace the master to correct. That a clue is, brake fluid turns dark fast after a flush. This happens IMHO, due to inner seal being damaged. Damaged from improper brake fluid or old fluid remaining way to long in the master.
Early in this thread, was posted a picture of the master plunger inner seal (o-ring). This seal (O-ring) is not replaceable form what I can tell. Nor does Toyota sell it. This inner plunger/seal, sit inside the master, forward of the master plunger with a seals that we can buy and replace. The poster was asking for someone that may have the inner plunger. Seems his mechanic broke shaft hold spring, while removing. I tried and failed, to remove this inner plunger.
My thinking is the seal leaks as pressure build in system. That the fluid leaks pass the O-ring at very high pressure. This is the screeching sound we hear. That this releases (leak) of brake fluid into reservoir under high pressure, are the bubbles we see.
Inner plunger with O-ring circled in yellow.
But if HP or LP within ABS unit switch bad, I would think you get a DTC & dash lights. Which I have seen and replaced the ABS unit to correct. But no screeching or bubbles that I knew of or reported to me, in that one.
I'm now of the opinion. That when we get the screech with air bubbles seen in reservoir. We must replace the master to correct. That a clue is, brake fluid turns dark fast after a flush. This happens IMHO, due to inner seal being damaged. Damaged from improper brake fluid or old fluid remaining way to long in the master.
Early in this thread, was posted a picture of the master plunger inner seal (o-ring). This seal (O-ring) is not replaceable form what I can tell. Nor does Toyota sell it. This inner plunger/seal, sit inside the master, forward of the master plunger with a seals that we can buy and replace. The poster was asking for someone that may have the inner plunger. Seems his mechanic broke shaft hold spring, while removing. I tried and failed, to remove this inner plunger.
My thinking is the seal leaks as pressure build in system. That the fluid leaks pass the O-ring at very high pressure. This is the screeching sound we hear. That this releases (leak) of brake fluid into reservoir under high pressure, are the bubbles we see.
Inner plunger with O-ring circled in yellow.
- Thread starter
- #400
Lets think about this. If the same MC would work for both LC and 4R it would save Toyota money to use the same one.
Next looking at the design, changing the springs for the Pressure Holding Valves and the Pressure Reduction Valves could easily give different pressure profiles which might be needed for the over 2000 lbs difference in weight. Or to put it another way an LC weighs almost 50% more than a 4R.
Unless you know all the ways the two parts can be different and have checked those out, I don't think substituting is a good idea.
It might seem fine in everyday braking, but in an emergency stop with ABS and/or VSC active and expecting a MC with certain characteristics it could be very bad.
