92 bonneville - front brakes locking up

[mnalep]

Hi,

I have a 92 bonneville. We've had a lot of repairs done, and the front breaks are locking up

while driving, after braking to slow or stop.

Some background: the brakes lost pressure, and we took it to a dealer. They ran ABS tests, and

this showed the ABS had problems (sensor, pump), but the reason for lost pressure was probably

master cylinder and rear brake seal. We had the dealer do the rear brake seal, which did not

fix the lack of braking pressure. We took car to PEP Boys to have master cylinder done (to save

money as dealer was so expensive).

We decided to do without ABS, as it would be up to $2,000 to repair it.

PEP Boys did the master cylinder, and replaced the brake lines which they said were leaking. We

now had brake pressure again, but after driving the front brakes locked up. (Either both or

one, we were not sure). Back to PEP Boys and they said the right front caliper was locking,

probably due to the cylinder in it, and said we had to replace both calipers and the flex lines

leading to them. We had that done but the front brake(s) continue to lock up.

Back to PEP boys and they now say they think it is a problem with the ABS system. We disagreed,

stating this happened after the master cylinder and lines were done, and the brakes never

locked up before. PEP Boys says it might be the ABS pump, which is dowstream from the master

cyclinder, and the BAS pump may be malfunctioning - but PEP Boys does not have the equipment to

test it!

I already know the ABS system was not functioning from the earlier dealer tests, but to

eliminate ABS as a cause, I pulled the fuse for the ABS system. The front brake(s) continue to

lock up. PEP Boys said to take car to a dealer and have them test it (PEP said they would pay

for the diagnostics, and redo any repairs PEP Boys did if the dealer says it is related to PEP

Boys work). So it seems we paid for calipers we did not really need.

I called the dealer, and they said to bring it in, but said it is unlikey that the ABS system

is causing the brakes to lock up. They don't know until I bring it in and they look at it - but

suggested over the phone that it might be related to the booster, because when the brakes lock

up one of the symptoms is that the brake pedal stays in the upright position, and is firm and

can't be pressed down.

Anyone have any tests or ideas as to fixing this problem? Does the booster seem like a culprit?

We are already out about $700 on all the repairs to date.

Thanks,
Matt

[LMP]

A problem with the booster control valves is a possbility. Were it for me, I would disconnect the vacuum line to the booster and plug it, and make carefully dosed brake tests in a known controlled environment. YOu do have brakes without the booster, but you have to press very hard, not safe enough for city driving. If the situation you describe happens regularly and stops with the booster disconnected, you would have your answer..

Another possibility would be a blocked return valve in the master cylinder....I have seen that a long time ago..heat expansion (after braking) has no place to return the fluid and thus pressurises the brakes instead...This can result from improper positionning of the brake linkage that does not return the master cylinder piston to the real rest position, thus blocking the fluid return path. Since they have worked on the master cylinder and the problem arose after that, I,m more and more inclined to think of this as the real culprit.

And amen for the gone ABS....

[mnalep]

Hi LMP,

How could I tell if there was a blocked return valve in the master cylinder? Do I open the master cylinder, after a lockup, and inspect the level of the brake fluid? Or the postion of some valve? Or positioing of the brake linkage. I am not sure hoe these parts are supposed to function or look in normal conditions, so I'm not sure what to look for under the abnormal brake locking condition?

Regarding your first suggestion, I'd be inclined to try that except for the fact that the brakes sometimes go days before locking up. I could be trying for a while, and that seems somewhat unsafe, unless maybe I jacked up the front end, and run the car while in a stationary position that way. I may ultimately try this, but I tend to agree that it seems related to the master cylinder.

My thinking is, why did the brakes not lock up before, over the last few years, with the same old ABS Pump. Seems unlikey that the ABS pump failed simultaneously with the Master cylinder.

So, if you know how I could inspect the master cylinder, and at what it is supposed to look like at rest, versus what I would be looking for after a lock up, that might really help me diagnose this.

Thanks,
Matt

[mnalep]

Well, I took it to the dealer, and they said the ABS pump was bad and causing the brakes to lock. The dealer put a new ABS pump in.

A few days later, the brakes start locking, with the brake pedal hard as a rock in the upright position again.

Back to PEP boys, and they put a 2nd Master Cylinder in. All seemed well for a few weeks, and the brakes locked again. So PEP boys put a 3rd master in, and again all was well for a week.

Now they are locked and the car is again at PEP boys. The manager says he will get a genuine GM master cylinder, and put that in.

Anyone have any tests or ideas as to fixing this problem?

Does the booster play a role in the dard brake pedal? I guess I don't see how the brakes locking and hard brake pedal in the upright position could exist at the same time. Seems like if the brakes are locked, the pedal should be down, or able to be pressed to the floor?

We are already out over $1,000 on all the repairs to date. This would be tolerable if the problem were fixed.

Thanks,
Matt

[vjolou]

Guess what, my daughter just had the same identical problem with her 2000 ply voyager. Intermittently after braking the front brakes would clamp down to a hard drag on the rotors and overheat. My son-in-law changed the brake fluid as we thought something was keeping the fluid to depressurize after breaking but this did not help. We thought it might be a master cylinder problem or the ABS also, but no one had worked on the master cylinder to get the return valve out of position to block return fluid and we didn't know diddly about ABS so we took it to the dealer. The dealer said he did a power bleed of the brake system because he said he didn't find any component bad except for air in the system. As of three days now, the brakes have not locked up! Before it only took about 10-12 miles on a cold set of brakes for the problem to occur and my daughter has driven that distance many times now. I've never heard of air doing this and I'm an old back yard mechanic. The brakes were hard as the bonnieville was reported to be, not spongy as one generally finds with a system with air in it?????? Go figure.

vjolou