Intermittent brake boost issue?

[tech_dog]

Description

o Sometimes my brakes feel "slow." They won't "grab" and I can't lock up the tires. The pedal feels a bit hard when this happens, and I use more pedal force than normal.

o The problem most often occurs when I first start driving, but also occurs randomly when running warm.

o I never have a soft pedal.

o I pass the test where the pushed brake pedel sinks when you start your engine.

o It is not linked to heavy braking.

o Its most noticeable at slow speeds. It's that last litle "grab" that's missing when you want full braking.

o It's a 94 LC with 120K miles and 33" tires.


Guesses

o Contaminated brake pads? I did spray PB Blaster in the same general before my lift, but I was careful not to hit the brakes. This probably isn't it because the brakes are usually good. They'd always be bad if they were contaminated.

o Brake boost failure in progress? If this is the case, then why does my pedal sink when I turn on the engine after pressing the brake pedal?

o Vacuum leak? If it were a slow leak, I'd expect to never have the problem after freeway driving.





Any suggestions?

 

[rockclmbr]

A lot of us have experienced this and replaced various components to no avail. Search around and you will find my research on this topic over and over. In the end, the best thing I did was have a professional shop bleed my brakes.

 

[Rkroelin]

Some more brake booster checks from the TSRM.

"1. OPERATING CHECK

(a) Depress the brake pedal several times with the engine off
and check that there is no change in the pedal reserve
distance.

(b) Depress the brake pedal and start the engine. If the pedal
goes down slightly, operation is normal.

2. AIR TIGHTNESS CHECK

(a) Start the engine and stop it after 1 or 2 minutes. Depress
the brake pedal several times slowly. If the pedal goes
down furthest the 1st time, but gradually rises after the
2nd or 3rd time, the booster is air tight.

(b) Depress the brake pedal while the engine is running, and
stop the engine with the pedal depressed. If there is no
change in the pedal reserve travel after holding the pedal
for 30 seconds, the booster is air tight."

Try them all. My booster is making sucking noises and passes them all save for the last.

 

[tech_dog]

Some more brake booster checks from the TSRM.

"1. OPERATING CHECK

(a) Depress the brake pedal several times with the engine off
and check that there is no change in the pedal reserve
distance.

(b) Depress the brake pedal and start the engine. If the pedal
goes down slightly, operation is normal.

2. AIR TIGHTNESS CHECK

(a) Start the engine and stop it after 1 or 2 minutes. Depress
the brake pedal several times slowly. If the pedal goes
down furthest the 1st time, but gradually rises after the
2nd or 3rd time, the booster is air tight.

(b) Depress the brake pedal while the engine is running, and
stop the engine with the pedal depressed. If there is no
change in the pedal reserve travel after holding the pedal
for 30 seconds, the booster is air tight."

Try them all. My booster is making sucking noises and passes them all save for the last.

I pass both of these tests.

 

[tech_dog]

A lot of us have experienced this and replaced various components to no avail. Search around and you will find my research on this topic over and over. In the end, the best thing I did was have a professional shop bleed my brakes.

Since there is no softness at all, I don't think it's an issue with air in the lines. I found a lot of people complaining of soft brakes, but did not find anyone with the symptoms I described.

I'm thinking it's got something to do with the vacuum line or the boost, but am still hoping that someone who knows the system better than I do can come up with a "that makes sense" diagnosis.

T.

 

[SR.GRINGO]

I have a 94 as well--I recently replaced my fluid and had it bled. This helped a little. I also replaced all 4 rotors with new pads--this helped a bit too but still leaves something to be desired. I replaced my axle to frame lines with Slees braided lines. My next step is to change the axle to caliper lines with slees version--some here on mud recomend this--I think beowulf posted something about this a while back...... I can stop pretty fast--I just dislike the soft peddle feel.

 

[pigbuggy]

I had a lot of the same problems a month or two ago. I never did any of the tests so i cant say that i have the same problem. I spent a while trying to figure it out, and when i first looked at the brake booster and tested it, it looked just fine. I got fed up with it, and after taking it to a couple mechanics, i said 'screw it,' and took it to a toyota dealership. they were as confused as I was, so they just decided to start replacing something until it was fixed, starting with the brake booster. I'm glad they decided to start with the $900 part, and not the other way around. turns out it was the booster.

I wont act like I know what I'm talking about, so i'll just give you something along the lines of what they told me. something inside the brake booster itself was cracked, and because of it, thats why things weren't right. Those weren't the exact words they used, but i thought i'd spare you the long ramble they gave me.

Bottom line is, that if you cant fix it, and you don't know what it is, and nobody else does either: just take it to somebody who does. (toyota dealership) say "look, I have a problem with my brakes..." (describe in detail what's wrong) Give them they keys, and let them deal with it. There is no use guessing with something as critical as brakes only to not have them work a couple weeks down the road when you really need them. In my case, it will most likely cost you an arm and a leg, but I felt better knowing that somebody was able to definitively pinpoint the problem and fix it properly. For that, (to me) it's worth it. Good luck!

 

[tech_dog]

I had a lot of the same problems a month or two ago. I never did any of the tests so i cant say that i have the same problem. I spent a while trying to figure it out, and when i first looked at the brake booster and tested it, it looked just fine. I got fed up with it, and after taking it to a couple mechanics, i said 'screw it,' and took it to a toyota dealership. they were as confused as I was, so they just decided to start replacing something until it was fixed, starting with the brake booster. I'm glad they decided to start with the $900 part, and not the other way around. turns out it was the booster.

Thanks. This is helpful. I think I'm about to go the same route, with a final inspection of the vacuum lines followed by replacing the booster.

 

[JasonH]

Bump. Did the new booster fix it? I'm having the same symptoms...

 

[ttubb]

Thanks. This is helpful. I think I'm about to go the same route, with a final inspection of the vacuum lines followed by replacing the booster.

X2 on the problems. Did a new booster fix your problems?

 

[JasonH]

It did for me. The pedal was hard and would actually push back against me some. I had the booster rebuilt at NAPA and it works great now.

 

[60wag]

There is a problem that shows up in the 60 series but I don't know if anyone has ever experienced in an 80: The booster vacuum is supplied through rubber hose attached to the intake manifold. The barb fitting threaded into the manifold becomes restricted with crud and makes the booster slow to recover after the first press of the brake pedal. It feels like you've lost the power assisted braking. I'd check for good vac plumbing between the booster and the manifold before replacing the booster.