no brakes!

[rtabish]

the braking in my car has been very soft...even mushy to almost nonexistent, since i bought it. it did, however, stop when i managed to have time to pump the brakes a bit, so i figured it was the master cylinder.

today was a pretty nice day, so i decided to replace the MC. it turned out to be a bit more convoluted than i was used to, since all of the MCs i have replaced in the past were standard old school one-piece units...but i got the job done, bled out the brake system with new fluid, and with the system done and sitting next to the curb, i had pretty good brake pressure. confident i had fixed the problem, i took off down the road to try it as speed. upon approaching the stop sign at the end of the block, i went to hit the brakes, and the pedal went all the way to the floor!!

with absolutely no brakes, and cross traffic at the intersection, i had to dive for the curb to stop the car. once the car was stopped, how ever, i could pump the peddle and get brake pressure back again. i limped the car back to my house and got out to figure out what the HECK i missed or forgot to check...fluid was up, didn't see any leaks or anything else....

i know next to nothing or even less about ABS and all the plumbing that goes along with it on these cars, but what i have read on the subject here, most people experience TOO MUCH brake if there is a malfunction in the ABS system.

anyone here have competent knowledge of the brake systems on these cars to have an idea what i might be experiencing...before i end up having to to take this to [and pay for] a local "expert"? much appreciated, if only to give me an idea of what i am looking at.

[00Beast]

First off you should go find a gravel lot or unpaved road and go actuate the ABS a few times, to get any/all air out of that system.

Have you bled at the calipers? Did you bench bleed your master-cylinder before you installed it? If all these answers are yes, I'd plug the vacuum line going to the booster and go test them out. If the brakes work well (albeit hard to push), then your vacuum booster is dead. If they're the same, there's another issue.

[rtabish]

i did bench bleed the master cylinder in the car with the supplied plastic fittings and tubes before hooking it up to the system. i thought i had bled the system sufficiently...at both the front calipers and the rear drums, but could give it another shot.

i followed the same procedure i use every time i bleed a system, but since the brakes were "mushy" before i changed the MC, i wonder if another component might be malfunctioning, and might not have even had to change the MC.

as for finding gravel to try to work out the ABS, the nearest dirt road is several blocks away, and i am hesitant to take the car that far with the potential of not being able to stop.

for what it is worth, the disk pads look to be brand new, and while i didn't pull the drums to check them out, i have not heard any metal-to-metal grinding or squealing from the back brakes when it was stopping. the only wild card in the system i can think of for blame is the unit under the air cleaner housing with the proportioning valve...which i assume is the ABS unit.

[1oldman]

Just a thought,

my '92 is known for the rear wheel brake cylinders leaking and failing. They are usually shot by 50k miles. Been like that since the car was new. After market brake cylinders usually don't last that long for me. - BC

[rtabish]

if the brake cylinders were leaking, i would think this would be apparent. the housings are dry and there is no fluid loss that i have noticed. if the brake cylinders were just plain bad, it would explain some of the mushiness but not the complete failure of the system to stop the car while in motion, but still provide good pressure sitting at the curb.

can i simply disable the ABS to see if that doesn't prove one way or another if that unit is at least a good point to start troubleshooting? i understand there is a PVM on these that tend to cause problems.

[00Beast]

You can disable the ABS either by disconnecting a wheel speed sensor or unplugging the module, but if there's air trapped in the module it will hurt you more than help.

[J Wikoff]

Pull the rear drums off and check out the big horseshoe shaped return springs. Make sure they are seated properly, and that they are supplying retracting force. I worked on my rear brakes this summer, and I either didn't get one back on right, or it popped loose. But I lost most all my braking pressure because of it. Replacements are cheap, BTW.

[rtabish]

got to re-bleed the brakes again. did get a bit better pressure, but not much. was talking to the counter guy at the local parts store earlier today, and he said that when he did his grand prix master cylinder he had to bleed out the system 3 time before he finally started getting decent feedback.

as big a PITA dealing with ABS is, i am surprised there isn't a kit or something for bypassing the whole thing in favor of standard brakes. i can see where they might be useful for some people in some situations, but i have managed to drive for many years without the need of a "nanny" backup system to protect me from panic stops. weather is turning here in Montana, and i will soon need to drive this car in the snow and ice...and if i can't trust the brake system, i may be in big trouble.

[LeSabre in Buffalo]

Have you checked the brakes with the brake booster disconnected? I had the EXACT same symptoms. It wasn't the MC, but the brake booster that went bad. The car stopped just fine with the booster disconnected, and didn't stop at all with it connected.

It's an easy check. Unplug the vacuum hose running to it, cap it, and try the brakes. If the pedal gets high and hard to press, you found your issue. Re-connect it and see if your issue returns once the car is started. That's the best way to tell what's really the culprit.

[rtabish]

00Beast said the same thing...no, i haven't tried that yet. interesting that this might be the issue, though. i would think that the brake booster would either work or not work, but not have that big an effect on the actual stopping pressure the brakes make. i would think it would just be harder to apply the brakes, not loose the brakes all together.

[LeSabre in Buffalo]

He's saying that because my old Buick did the same thing and it wasn't the MC or any other hydraulic component. It's a rare problem to have. While the booster doesn't go bad very often, it can behave exactly as you have described your brakes.

The ringer for me that you NEED to check your booster is when the car was sitting, presumably with the engine off, the brakes were fine. Once the engine started, supplying vacuum to the booster, the brakes went away. My car had those EXACT symptoms.

[red454]

I am having similar problems on my 93 SSEi. Keep bleeding the brakes and they seem to be ok for a short while, then mushy pedal. I will try the booster suggestion tomorrow.

How hard is the booster to change?

[rtabish]

the booster isn't too tough to replace. the toughest part is getting up under the dash behind the brake peddle to undo the 4 mounting bolts and brake peddle linkage, and taking the master cylinder off in the engine bay. probably have to take off the strut tower brace as well. i don't like doing it because i hate trying to squeeze and contort my 6'4", 300lb body around trying to get up under the dash...for any reason.

and i got a chance to pull the booster vacuum hose on mine and try it out. looks like Monday is another trip to the parts farm.

[red454]

the booster isn't too tough to replace. the toughest part is getting up under the dash behind the brake peddle to undo the 4 mounting bolts and brake peddle linkage, and taking the master cylinder off in the engine bay. probably have to take off the strut tower brace as well. i don't like doing it because i hate trying to squeeze and contort my 6'4", 300lb body around trying to get up under the dash...for any reason.

and i got a chance to pull the booster vacuum hose on mine and try it out. looks like Monday is another trip to the parts farm.

Well, I am 5'6" and 150 lbs, and I don't like getting up under there, so I can't imagine what it is like for you... Anyway, post your results when you can.

But isn't disconnecting the vacuum line the same as just sitting there with the engine off and pushing on the brake pedal?

[LeSabre in Buffalo]

So the booster's shot on yours?

To replace, the strut tower brace does need to come off. The M/C can be shoved far forward with no ill effect. Well, as long as the reservoir cap is tight. You'll want a nice flat-head screwdriver or two for the clip on the pedal linkage. It can be bent back into shape when it's time to re-install.

Don't worry about the 4th bolt in the upper left-hand corner that's an impossible nightmare to replace. I left it off with no ill effects. To get it out I needed a wobble and an extension on my ratchet. More extensions would be nice to remove it.

[rtabish]

i think the idea is to get the engine vacuum up. with my engine idling and the car sitting still at the curb, i can get brake pressure, but as soon as i put it in gear and start rolling [raising the engine vacuum] the brake pressure seems to drop off a lot.

if it weren't for the ABS...the only part of the brake system i am not used to dealing with, troubleshooting why i am having problems would have been easier. my next question would have to be, if the booster isn't the problem, what is the next step?

[J Wikoff]

Have you pulled the rear drums yet?

[rtabish]

haven't been back under the back end of the car since last week, so i haven't looked at the drums. since i get to do any automotive work i have to do outside on the street in front of my house, getting a window of opportunity [when both weather and other obligations allow me to] to get into even minor R&R jobs, takes timing.

at least i have a few clues now. thanks, guys.