Weird Brake Failure Issue (1 Viewer)

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swampfoxSC

swampfoxSC

Joined
Apr 30, 2012
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I have an extremely odd brake issue. It started out a couple of times when I went to hit the brakes it went to floor with little resistance and zero stopping. I quickly pumped the pedal and had full strong braking. As you can imagine this was quite alarming. I checked everything I could and found some questionable vacuum lines. So I replaced everyone on the truck. A few days go by and it seems to have fixed it. Then it happened once again. I noticed that it was happening at slow speed low RPM thankfully. So I started digging again I initially thought maybe the booster went bad again. I had replaced it about two years ago. But it was passing all the normal testing. Then I was thinking an intermittent issue with the check valve. So I swapped in a new one and immediately was able to get the failure. Then this got me thinking it has to be something else. Especially since every booster failure I have every experienced resulted in a very hard pedal. Then the fact that the pedal is going through full travel to the floor with little resistance. Well yesterday I figured out how to replicate the issue every time. If I slowly apply the brakes it will go to the floor. If I apply the brakes at a moderate to fast speed it works as it should. Now this has got me thinking something is wrong with the master cylinder. Is there a way the seals could fail to cause this? It’s really about the only thing I can think of at this point. It kinda makes sense that the extra speed would build pressure fast enough to push against the seal and make it work. Any of y’all have any thoughts or experience with such a issue?
 
I've never seen or head of that before, but is makes sense that the master cylinder seals are at fault.

FWIW, there's only one vacuum line that matters to the brake booster. Replacing all of them isn't a bad idea, if they're old, but unless they were all off the manifold, I can't see you losing enough vacuum to influence the booster, and even then, I don't think that would happen, although I've never tried it.
 
Malleus said:
I've never seen or head of that before, but is makes sense that the master cylinder seals are at fault.

FWIW, there's only one vacuum line that matters to the brake booster. Replacing all of them isn't a bad idea, if they're old, but unless they were all off the manifold, I can't see you losing enough vacuum to influence the booster, and even then, I don't think that would happen, although I've never tried it.
Click to expand...
Yeah I just went ahead and freshened them all up while I was at it. I ordered some of the formed hoses that I had replaced with bulk stuff a while back
 
FWIW, I don't think you could lose substantial vacuum through the small vacuum lines, at least not enough to make a difference at the brake booster. A gasket leak between the throttle body and manifold, or between the upper and lower, yes, but there doesn't seem to be enough surface area in the few penetrations in the manifold for the small tubing to create a significant loss at operating engine rpm. Again, maybe at idle, but you're really stopped at idle already.

I'd bet against the brake master cylinder. Hydraulics are a black art, so there's no real way to tell, unless you've got full instrumentation at every joint, seal, etc, which you don't. You don't even have this at the wheels, even with all the computers on the newer models.
 
Did you check all the brake lines? Could a bulging/bubbled break line cause this? I really don't know much about brakes, just thinking out loud.
 
Just spitballing, but...
it would seem that if the flexible hoses were at fault, the failure would be the other way around; that is, a slow pedal would create sufficient pressure abut a fast pedal would cause immediate bulging, but I could be wrong about that. I've seen some unbelivable magic happen in hydraulic systems.
 
leonard_nemoy said:
Did you check all the brake lines? Could a bulging/bubbled break line cause this? I really don't know much about brakes, just thinking out loud.
Click to expand...
Keep thinking out loud I’m outta of thoughts and am about to throw a master cylinder at it
 
Couple things
1) if you lose vacuum to booster, the pedel is ROCK HARD and it takes just about all ones strength to get the truck stopped.
So that's not the problem, but its good to change the vacuum lines once in a while anyway.

2) If your pedle suddenly goes to the floor and you can't pump it up, there is a catostophic leak or blow out. Doesn't sound like your problem.

3) If your pedle starts getting lower and lower toward the floor and you can pump it up, sort of, but the brakes still feel underpowered and mushy, you've been agressivly braking or riding the brakes and boiled the moisture out of the fluid and created a steam bubble, acts like air in the lines and cusions the hydraulics. Time to change the break fluid or possibly your driving habbits.

4) If your pedle goes to the floor and you can pump it up for that stop, but the problem comes right back, you have air in the lines or a leaky master cylinder seal. This sounds exactly like the problem you described.
I'd start with the master cylinder, and if the booster is the same age, might want to change that "while your in there".
 
^^^^^This^^^^
 
Rusty Marlin said:
Couple things
1) if you lose vacuum to booster, the pedel is ROCK HARD and it takes just about all ones strength to get the truck stopped.
So that's not the problem, but its good to change the vacuum lines once in a while anyway.

2) If your pedle suddenly goes to the floor and you can't pump it up, there is a catostophic leak or blow out. Doesn't sound like your problem.

3) If your pedle starts getting lower and lower toward the floor and you can pump it up, sort of, but the brakes still feel underpowered and mushy, you've been agressivly braking or riding the brakes and boiled the moisture out of the fluid and created a steam bubble, acts like air in the lines and cusions the hydraulics. Time to change the break fluid or possibly your driving habbits.

4) If your pedle goes to the floor and you can pump it up for that stop, but the problem comes right back, you have air in the lines or a leaky master cylinder seal. This sounds exactly like the problem you described.
I'd start with the master cylinder, and if the booster is the same age, might want to change that "while your in there".
Click to expand...
Booster was changed about two years ago. The master cylinder has 322,000 miles of pumping. I’m thinking it’s the master cylinder. I can make it fail every time by very slowly depressing the pedal. It works fine if applied with any sort of quickness.
I may try flushing with new fluid first for s***s and giggles just to rule that out. But I’m leaning toward it being the master
 
Just as another data point, I had the same exact situation as you about a month ago. I postponed my flush to “when I get time from work,” and just managed to pump the brakes to build enough pressure to stop. As the days progressed though, I could feel the brakes getting worse, and even more when turning. Finally I did a flush with an experienced friend and felt no improvement. Checked out my brake pads and they were still great, but one rotor had a gouge in it, so I posted it to Mud and someone mentioned it may have been a pebble getting caught. Replaced that rotor and both rear pads as the one with the bad rotor also had a small gouge in the pads. Still nothing.
I finally bit the bullet and bought an Aisin master cylinder, watched a couple of videos, read a couple of posts, and swapped it out. When I bench bled the new master and test bled the old master, there was a world of difference in the amount of pressure I needed to push the rod in to get fluid moving. I’ve read somewhere that our master cylinders have two spots where the fluid pressurizes when brakes are pushed, and some small rubber gaskets/seals are the only thing keeping them separate. I assume the first part of my cylinder was compromised because it was almost effortless until I got to the second half of pushing the rod in where I felt any resistance, and even then it wasn’t much.
As soon as I slapped in the new master, and bled the system, the braking felt insanely different! I always thought our cruiser’s brakes were just somewhat weak because of the weight these guys carry, but man is it scary good with how the slightest tap on the brake pedal causes it to immediately start slowing down!
Sorry I don’t have many of the technical terms down as I’m still learning every time I work on something, but I really hope this helps as I understand the frustrations that come with this situation!
 
The '95-'97 Land Cruiser/'96-'97 LX450 master cylinder is still available from Toyota. The '93-'94 master cylinder is NLA, although you can buy "OEM style" replacements from aftermarket sources, like RockAuto.

The difference between the two styles is that the earlier style has the fluid level sensor in the reservoir, the later style has the fluid level sensor in the cap. They are functionally the same.

If it was me, and I was going to replace the master cylinder, I'd flush the old fluid first, with the old master cylinder in place. This way, there's no chance any old contaminated fluid will be in the new master cylinder when it's installed. All the new fluid can stay in place while you install the new master cylinder, since it's at the top of the fluid column.

You shouldn't have to bleed the system after you bench bleed the master cylinder, as per the FSM. The emergency plugs that @NLXTACY has on the Absolute Wit's End site are ideal for this job.
 
swampfoxSC said:
Where did you get your aisen master cylinder from?
Click to expand...
I got mine from Cruiser outfitters, but I believe I checked rock auto after I ordered and it was cheaper on there.


Malleus said:
The '95-'97 Land Cruiser/'96-'97 LX450 master cylinder is still available from Toyota. The '93-'94 master cylinder is NLA, although you can buy "OEM style" replacements from aftermarket sources, like RockAuto.

The difference between the two styles is that the earlier style has the fluid level sensor in the reservoir, the later style has the fluid level sensor in the cap. They are functionally the same.

If it was me, and I was going to replace the master cylinder, I'd flush the old fluid first, with the old master cylinder in place. This way, there's no chance any old contaminated fluid will be in the new master cylinder when it's installed. All the new fluid can stay in place while you install the new master cylinder, since it's at the top of the fluid column.

You shouldn't have to bleed the system after you bench bleed the master cylinder, as per the FSM. The emergency plugs that @NLXTACY has on the Absolute Wit's End site are ideal for this job.
Click to expand...
I agree with Malleus. I bled the fluid with the old master cylinder in place, but because I believed it would help the brakes, which it didn’t. I only bled the brakes after on the new system because I didn’t have plugs in my lines that connect to the master. The new one did come with some caps, which I used, but I wasn’t 100% sure it kept all the air out, so I bled the brakes again to make sure no air was trapped. The brakes feel so much better now!
 
Curious, can the master cylinder be rebuilt with new O rings?
 
Swirvinnirvinn said:
I’ve read somewhere on here that rebuilding it isn’t worth it and I believe it was either beno or someone as respected on the forum, so I just went with the Aisin part.
Click to expand...
the cost differential between the rebuild kit vs a replacement isn't that much, so I can see that point. Cruiseroutfitters have both so it was an easy comparo.
 
alia176 said:
the cost differential between the rebuild kit vs a replacement isn't that much, so I can see that point. Cruiseroutfitters have both so it was an easy comparo.
Click to expand...
You will also need a brake cylinder hone and hone the primary bore on the MC before you install the new kit to remove any pits or gouges. If you've never done this before, just buy the new unit unless you're Jonesing for a project. It's not hard, but you can put in the work and still have it fail.
 
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