Clunk / Sinking brake pedal (1 Viewer)

[Reinhardtius]

Hi Folks,

Check out this video in which I show you my brake pedal exhibiting some unusual sinking behavior. When you press on the pedal, it gets firm, but then at the bottom, in sinks down further. This happens while driving too, resulting in consistent braking, then much more aggressive braking when the pedal sinks downward.


Are my seals going in the brake master cylinder, or something? If so, which parts should I get together? It's an 88 BJ75.




Thanks,

Reinhardtius

 

[jblueridge]

If you have rear drum brakes, take them apart and look for leaks.
Brake fluid from a leak mixed with brake dust makes for weird things happening.

I eventually followed advice from others here and replaced original master cylinder with one from a 1990 FJ80 and the same swap with the vacuum booster. Replaced front calipers and pads with slightly larger ones off a 1990 4runner.
It required some fiddling to make it work.

 

[Reinhardtius]

If you have rear drum brakes, take them apart and look for leaks.
Brake fluid from a leak mixed with brake dust makes for weird things happening.

I eventually followed advice from others here and replaced original master cylinder with one from a 1990 FJ80 and the same swap with the vacuum booster. Replaced front calipers and pads with slightly larger ones off a 1990 4runner.
It required some fiddling to make it work.

Understood. I'll have to take a look. I went ahead and just ordered a new OE replacement brake master cylinder.

Anyone else have a hunch that this clunking/sinking is probably a failing brake master cylinder? Just looking for assurance or alternative hypotheses.

Cheers,

Reinhardtius

 

[roscoFJ73]

No one can say for sure if its the M/C , but it most likely will be. Any leakage in the brake system will cause the pedal to drop. Ive had the brakes working perfectly on these models just using the correct parts for 7* series.

 

[Reinhardtius]

No one can say for sure if its the M/C , but it most likely will be. Any leakage in the brake system will cause the pedal to drop. Ive had the brakes working perfectly on these models just using the correct parts for 7* series.

Replaced the MC and the sinking pedal persists, particularly once the truck has been driven for some time.

I am going to test the brake booster and check valve next. I was thinking that the sudden increase in braking bite could be a sticking caliper, but the truck does not pull to one side or anything. My hunch is that it is vacuum related. What do you think?

 

[jblueridge]

When my rebuilt booster was failing I had no brake assistance from it...pedal was like a brick wall.
After MC and drum brakes, I’d suggest checking the rubber brake hoses. They can simply swell as well as leak.

 

[roscoFJ73]

My hunch is that it is vacuum related. What do you think

My experience says its not. Lack of vacuum makes the pedal go hard. Disconnect the vacuum to the booster and try it. It should go rock hard after 1-2 pumps.
Are you sure there is no air in the brake lines?

 

[Reinhardtius]

When my rebuilt booster was failing I had no brake assistance from it...pedal was like a brick wall.
After MC and drum brakes, I’d suggest checking the rubber brake hoses. They can simply swell as well as leak.

The pedal is not like a brick wall, so that is rather good news. What about the drum brakes would you suggest checking on? I could just replace the rubber brake hoses. They do look a little old. I just want an explanation for why the pedal would do this if something was not right about the hoses. What's going on there?

My experience says its not. Lack of vacuum makes the pedal go hard. Disconnect the vacuum to the booster and try it. It should go rock hard after 1-2 pumps.
Are you sure there is no air in the brake lines?

I am pretty confident that there is not air in the lines, but I can bleed again. I waited 10-20 seconds after seeing a bubble before fastening the bleeder nipple and moving to the next wheel.

 

[jblueridge]

Get the brake drums off and look at the hydraulic wheel cylinders, the little gizmos the brake lines are connected to. Look for leaks.

How about that Load Sensing Proportioning Valve above the rear axle? It has a bleed nipple on it too. I replaced mine but they can be dismantled and cleaned. I believe the Toyota part number is 47910-26040

A truly bad brake hose would look truly bad. Cracks in the rubber and so on.

 

[Reinhardtius]

Get the brake drums off and look at the hydraulic wheel cylinders, the little gizmos the brake lines are connected to. Look for leaks.

How about that Load Sensing Proportioning Valve above the rear axle? It has a bleed nipple on it too. I replaced mine but they can be dismantled and cleaned. I believe the Toyota part number is 47910-26040

A truly bad brake hose would look truly bad. Cracks in the rubber and so on.

I am going to look more closely at the rear brakes as soon as I get the time to get in there. Maybe something is up in there. As I mentioned in a new thread (Rear drum brake service? Educate me), there rear is humming when I brake, so maybe something is stuck or needs servicing back there.

 

[jblueridge]

Sometimes it is just brake dust that is trapped in there, but once I had a goo of dust + brake fluid from a leak inside and my brakes grabbed like crazy on that on side. Thought a bearing had seized or something.

 

[Reinhardtius]

OK, six years later and this issue has finally been diagnosed. I rebuilt the rear wheel brake drum cylinders, and I also redid the rear brake hardlines. That, and at the same time, I also replaced all of the bleeder bolts on all four corners. What I have noticed after performing probably 6-7 brake bleeds with a pressure bleeder is that the LSPV appears to be leaking slightly. It is also noticeably stuck in a few positions.

I just ordered a replacement LSPV and my expectation is that this will fix the issue once and for all. After all, there is nothing left in the braking system that I have not once-overed.

 

[Reinhardtius]

OK, and update. I have replaced the LSPV and I have tied the long rod that connects to the axle to the top of the bed.

Unfortunately, after bleeding (6 or 7 times) all 5 bleeder nipples (starting with LSPV bleeder, then pass rear, then driver rear, then pass front, then driver front) with power bleeder, the pedal still goes soft. If you pump it twice, it is very firm, but goes soft again after about 6 seconds. And, I also notice, unlike before, that when you really push hard on the pedal, the brake pedal does not collapse like it used to with the old LSPV installed.

I am not sure where air is stuck, or getting in. I do not observe fluid leaking from anywhere.

My next thinking is to find some flare nuts that are the exact side of the brake lines, and cap off each corner to isolate the source of the air or the leak. Is that something anyone has experience doing? What is the fastest way for me to identify and resolve this brake issue?

 

[jblueridge]

Is the pedal feel any different when you strap the LSPV rod to the axle (in the fully down position)?

If I am thinking straight, when the arm is strapped up, that simulates a huge weight load in the rear end so the lspv should send fluid or allow more pressure to go to the rear brakes. Normally, the front discs do most of the work.

 

[coldtaco]

why did you start bleeding with the LSPV? when I replaced mine I started with the pass rear, then driver rear, then LSPV, then pass front, driver front. I didn't move the LSPV arm any just left it where it was.

 

[Felde]

Is the pedal feel any different when you strap the LSPV rod to the axle (in the fully down position)?

If I am thinking straight, when the arm is strapped up, that simulates a huge weight load in the rear end so the lspv should send fluid or allow more pressure to go to the rear brakes. Normally, the front discs do most of the work.

This is not how the LSPV works.
When the vehicle is level (moving without braking yet), full brake goes to the rear. When during braking the vehicle sinks in on the front / rear goes up (can be simulated by lever down), the LSPV gradually sends more brake power to the front, using a bypass line.
When the vehicle is level / LSPV is full open to the rear, this (then sealed off) bypass line belongs to the front brake circuit; means: If there is air in that line, brakes feel spongy. This line can only be bled to the front, with LSPV open = lever down. It is a long line, so lots of fluid and patience is needed.
Also note that the LSPV operates by pressure to open. Bleeding devices sucking may thus not work well.
Good Luck Ralf

 

[Reinhardtius]

What would happen if I had flip flopped the two brake lines entering the LSPV? Could this be the cause of this issue?

If @Felde is correct, then I need to *not* have the LSPV tied to the bed, and instead need to have it pulled downward.

 

[Reinhardtius]

What would happen if I had flip flopped the two brake lines entering the LSPV? Could this be the cause of this issue?

If @Felde is correct, then I need to *not* have the LSPV tied to the bed, and instead need to have it pulled downward.

I guess the other relevant question to ask is which one of the two brake lines is the return on the LSPV and which Ray Lyne exiting the master cylinder is the return? Maybe I do have them flip-flop but I have no idea and I can’t see any documentation in the chassis manual about which is which.

 

[mattressking]

I see a lot of back and forth but nothing useful here.

1. Have you actually looked under the truck for leaks? Is the master cylinder full?
2. Have you bled the brakes furthest to closest and confirmed no air? I see you bled but nothing about zero bubbles coming out of each corner.
3. Like @roscoFJ73 said, sinking pedal means its not rock hard which usually rules out booster leak. You can however test it, download FSM and it has the procedure.
4. Do above then get back to us.

Sidenotes:

A. Does your parking brake work correctly? If not, that's a quick way to tell if your rear drums are adjusted correctly.
B. If you pump pedal does it firm up? If so, see 2 above. EDIT: I see it does, all simple points come straight back to air in system. Did you recently do any brake work?
C. Skip the power bleeder, go old fashioned and bleed with a bottle and hose, fastest and easiest method to confirm lack of air.

 

[Reinhardtius]

I see a lot of back and forth but nothing useful here.

1. Have you actually looked under the truck for leaks? Is the master cylinder full?

Yes, I have, and yes, it is full.

2. Have you bled the brakes furthest to closest and confirmed no air? I see you bled but nothing about zero bubbles coming out of each corner.

Yes, I started with furthest. No bubbles present.

3. Like @roscoFJ73 said, sinking pedal means it’s not rock hard which usually rules out booster leak. You can however test it, download FSM and it has the procedure.

Ok

4. Do above then get back to us.

Did, and getting back to you.

Sidenotes:

A. Does your parking brake work correctly? If not, that's a quick way to tell if your rear drums are adjusted correctly.

Parking brake does not work. Needs adjusting. Just rebuilt rear brake shoes, drums, cylinders.

B. If you pump pedal does it firm up? If so, see 2 above. EDIT: I see it does, all simple points come straight back to air in system. Did you recently do any brake work?

Yes does firm up, but goes soft again after awhile.

C. Skip the power bleeder, go old fashioned and bleed with a bottle and hose, fastest and easiest method to confirm lack of air.

Why skip the power bleeder?