Brake experts??

[rbarton]

Hello, I have a non-Porsche brake problem that has been driving me nuts. The vehicle is a '94 Suburban 2wd with 4-wheel ABS. The issue is that the truck always stops fine, but the firmness of the pedal varies and about 50% of the time when sitting at a stop, the pedal will slowly sink toward the floor. If stopped at a light long enough, the pedal will go all the way to the floor and turn on the brake idiot light. Every time it does it reminds me that my wife says I'm an idiot for driving this thing.

Sounds like a fluid leak, right?? So I've replaced the rear wheel cylinders (one did look like it had been leaking at some point) and the front calipers. I've replaced the seals/grommets between the master cylinder and fluid reservoir and also the diaphragm in the reservoir cap. I've inspected and re-inspected the soft and hard lines. I've bled and rebled and finally paid the dealer to bleed the brakes to make sure all the air is out of the system. The master cylinder and booster were replaced when I bought the truck in an attempt to fix this problem as well. Now about all that I haven't messed with is the ABS controller, a huge contraption that both lines from the master cylinder go into and from which the individual lines come out of. In between I have no idea what this thing does. Could it be the source of my problem? Anything else I should check?

Thanks in advance for any help!!

[RoninLB]

I have no idea about ABS. If it was a routine system I'd start with a new master cyl.

[IROC]

I agree with Ronin... If you don't have any visible fluid leaks anywhere in the system, pedal movement like that is almost always the master cylinder.

You said it was replaced...but...I'd check it again anyway. I don't know much (OK hardly anything) about boosted brakes, but could this also be a leaking diaphragm in the booster? Since it sounds like you've addressed most everything else, I would go back and check the obvious things again.

Having said all of that, I think some of the brakes GM installs on their cars are simply evil. You might need to perform some sort of sacrifice to the friction god or something. I think I read that in a Chilton manual once...

Mike

[RoninLB]

Originally posted by IROC

, but could this also be a leaking diaphragm in the booster?

I usually check that by beginning with engine off, then press on brake pedal, then leave pedal loaded and start engine. A good booster will enable the pedal to drop slightly.. try it on a pb sys for entertainment.

[rbarton]

Thanks for the help guys. I checked the vac booster per Ronin's procedure and it seems OK. Is there a good test for the master cylinder other than just trying a new one? The master cylinder and booster were just replaced, but I know they were cheapo rebuilds from the corner auto parts store.

One thing I just figured out after a drive today: the falling pedal problem occurs only when I apply the brakes lightly for the first couple of inches of pedal travel. If I give the pedal a good firm stab upon initial application the pedal stays firm.

Thanks again for the help. I've learned more from this site in the last 3 months than I would have believed possible. I need to get this POS fixed so I can get back to the 911.

[RoninLB]

Originally posted by rbarton
The master cylinder and booster were just replaced, but I know they were cheapo rebuilds from the corner auto parts store.

maybe there is some kinda warranty?

sometimes you can't make a hard answer to what the prob "was" till after it's fixed. I had fun tracking down a non fluid leaking/ air leak in a caliper yrs ago.

[Jared at Pelican Parts]

I've had similar problems on my dad's 6.2L GMC Diesel.

Sounds to me like you've either got a bad brake booster or a leaking master cylinder.

When dealing with GM stuff, I've found that new units are almost always superior to the rebuilt ones. I would imagine that there is some sort of warranty on the rebuilt one. it's not that uncommon to get a defective rebuilt part.

On my dad's diesel, we had two rebuilt alternators fail before we just bought a new one and solved the problem.

[rbarton]

Unfortunately, I did not buy the master cyl. or booster; the guy I bought the truck from did. I think I'll bite the bullet and go ahead and price some new parts.

Thanks again guys.