Checking my thought process....brakes: Grassroots Motorsports forum: Grassroots Motorsports Magazine

Subscribe get a full year for only $19.95 Sample get a free issue—no strings attached

June 29, 2009 6:34 p.m. JThw8 SuperDork

Ok, so when I got the Tahoe the one major issue was the brake pedal was mushy and would drop as you held your foot on it.

No biggie, new master cyl. Pedal was still mushy and still dropped. I bled it, and bled it and bled it again. Then through some research I find the rear brake adjustment will definitely play a large part in pedal feel. So I manually adjusted them and it feels much better now, it will lock up the brakes which it really wouldn't do before

However, if you sit stopped and keep your foot on the pedal it will still slowly creep toward the floor. All I can figure is I got a bad reman, yes I did bench bleed it before installing. I can't think of any other component (other than a leak which I have checked for) which would cause that symptom. But I figured before I pulled it out and took it back for another, and went through the very painful bleeding process again I'd see what the other experts have to say.

Anything I could be missing here?
June 29, 2009 7:26 p.m. ignorant PowerDork

If it creeps to the floor, it's a bad master in my experience.
June 29, 2009 7:37 p.m. benzbaron Reader

When I did my toyota pickups brakes I had an issue with soft and mushy pedal even after using a power bleeder and adjusting the rear drum brakes. I had to have a helper pump the brakes and crack the bleeder to get the air out of the system.
June 29, 2009 8:17 p.m. JThw8 SuperDork

benzbaron wrote:

When I did my toyota pickups brakes I had an issue with soft and mushy pedal even after using a power bleeder and adjusting the rear drum brakes. I had to have a helper pump the brakes and crack the bleeder to get the air out of the system.

But this doesnt have a mushy pedal, after adjusting the rears the pedal is nice and firm (for a late 90s GM, but that's another story) Its firm but if you hold pressure it gradually creeps toward the floor. That's always said MC to me, Im just making sure it couldnt be something else before I return a "new" MC to the store and start over.
June 30, 2009 8:04 a.m. 16vCorey SuperDork

A slight leak in any part of the braking system can cause that, but you'd be loosing fluid. It could even be a very minor leak, so even if you're not leaving puddles, I'd mark the reservior and check the level a week or so later. If you're not loosing fluid, I'd say MC or ABS assembly. I'm not sure what year yours is, but I believe some of the early '00s GM trucks and SUVs had common ABS issues.
June 30, 2009 9:31 a.m. JThw8 SuperDork

16vCorey wrote:

A slight leak in any part of the braking system can cause that, but you'd be loosing fluid. It could even be a very minor leak, so even if you're not leaving puddles, I'd mark the reservior and check the level a week or so later. If you're not loosing fluid, I'd say MC or ABS assembly. I'm not sure what year yours is, but I believe some of the early '00s GM trucks and SUVs had common ABS issues.

I'm fairly certian there are no leaks. I have been over the whole system front to back and pulled the wheels looking for any leaks I could find. I drove it about 150 miles and no drop in the fluid level of the reservoir. With the amount of pedal travel I can get if I keep my foot on it the fluid loss should be noticeable if it was from a leak.

The ABS theory is one thats bouncing around in my mind. It does have ABS and I've read about the potential issues but I cannot find any specific issues which would provide the same symptoms.
June 30, 2009 9:52 a.m. sachilles Reader

Does it have a vacuum booster? Sounds like you might be losing the vacuum assist.
June 30, 2009 11:37 a.m. JThw8 SuperDork

sachilles wrote:

Does it have a vacuum booster? Sounds like you might be losing the vacuum assist.

It does have a booster but the pedal will sink even with the engine off, my experience has been that when the booster goes the pedal gets harder not softer/sinking. But more than one person on the fullsize chevy forums has claimed this as a fix despite the counterintuitive logic so it may be something to look at as well.
June 30, 2009 9:59 p.m. curtis73 HalfDork

Many of the aluminum GM masters had porosity issues. This means that the O-rings don't get a good seal in the bore and it could cause the problem you describe.

I would consider a new MC. The other option is a sleeved MC, but that's a lot of money.

Basically, if there are no leaks after the MC, and the pedal is creeping to the floor, it has to be the MC. Some ABS units (like the Keihin) could rarely leak into the ABS module and not show up as a leak until the ABS module was full and overflowing... but I've never heard that from a Bosch unit like yours would have.
June 30, 2009 10:15 p.m. Junkyard_Dog Reader

sachilles wrote:

Does it have a vacuum booster? Sounds like you might be losing the vacuum assist.

Maybe not so much losing it as gaining it, though if it happens REPEATEDLY with the engine off its probably not what I have in mind-which is check the check valve. A bad check or limit valve in the vacuum line going to the booster can cause exactly this problem. Also some ABS systems can cause this but further research would be required.

Or it could be the crappy GM brake pedal everyone complains about. Got a similar vehicle to compare with? Say at a used car lot specializing in ex-police cars? Come to think of it, do they have a shop containing a mechanic familiar with that vehicle?
July 1, 2009 8:06 a.m. JThw8 SuperDork

Thanks for the ideas and comments everyone.

Happens repeadedly - yes, the pedal will always creep toward the floor engine off or on. Its very slow (almost unnoticable) with normal pressure and slightly faster as more pressure is applied.

Crappy GM pedal feel - Im used to this having owned 3 suburbans prior. I took care of the worst of it with the rear brake adjustment and now the pedal feel is "normal" for a GM of this era.

The "new" MC I put in was a rebuild and is under warrantee so I think Im going to go ahead and exchange it and get a NEW not rebuilt MC and see what happens.
July 1, 2009 10:20 a.m. Jerry From LA Reader

Good idea. I bought one rebuilt master in my life and won't make that mistake again. If you're stopped at a light and suddenly the brake pedal drops with a pressure loss, you have a master cylinder problem. Buy a new one.