95 Tahoe brake pedal goes to floor

[95TahoeBoston]

Hello all -- Great forum!

I've got a 95 Tahoe 4x4 5.7.

The brake pedal has ALWAYS felt very soft on this truck from day 1, but with 150k+ miles it's done pretty well.
About 18 months ago, I started to get ABS activity (vibration in pedal, mooing sound (abs pump?) from under hood) when coming to a complete stop; I assume this is a bad sensor, perhaps in the transfer case.

When I removed the #18 fuse, the ABS light is on, the abs activity stopped. However, this also caused the truck to refuse to enter transmission lockup, and also the crusie control became inoperative. I assumed this is normal, as both of these systems could easily be tied into the brakes working properly.

So I put the fuse back in since my fuel mileage went so low without torque converter lockup.

About a year ago, I was visiting my brother, and he recommended a trip to Pep boys. They insisted that it must be a master brake cylindar and replaced it. There was of course no difference.

About 1 month ago, my brake pedal went to the floor. The truck still had *SOME* braking, but it just isn't safe. When coming to a complete stop, I still have abs pump on and vibration in the pedal, in fact it does pump up the pedal ever so slightly.

So this time I figure the PepBoys master cylindar is a POS so I replace it. I also completely flush and bleed the brake system at each wheel. The pedal still goes right to the floor, although it is somewhat firmer than before. There is no leak anywhere. With engine off the pedal is rock hard with no fade. With engine running and vacuum line removed from power boost the pedal is rock hard with absolutely no fade, and you can hear the back shoes activating with each push (they creak a little! )

I'm stuck on how to diagnose the problem. I'm not even sure I can imagine where the problem might be!

I suppose it could be in the ABS system, but how would I check that? There must be valves and such in there.

Could it be another part elsewhere in the brak system? Someone mention an apportioning valve? Where would that be in my truck, and how can I test for that?

Any help would be greatly appreciated!

Thanks,
-Ron.

[777stickman]

Could be the vavuum booster itself. To find the propotional valve follow the two lines from the master cylinder. This valve proportions pressure to the front and rear brakes. It also will shut off pressure to the front or rear if there is a massive leak in one of the systems. As far as the ABS, you need to get it scanned for DTC's to see if there is a problem with that system. Also did you bench bleed the new master cylinder?...........Steve

[95TahoeBoston]

Hi Steve and thanks for the quick reply!

How could it be the vacuum boster itself? I've NEVER had one fail before where the pedal didn't just get hard; why would the pedal go to the floor if it was bad? How can I tell if there's something wrong with it (or the check valve, if there is one, maybe)?

Also, I can't imagine it actually being the valve -- wouldn't the pedal stay hard no mater to which half of the system the pressure was being sent?

I really hate tossing parts at a problem like I did with the Master.

Thanks again, Steve!

[777stickman]

The 1st thing I would have replaced is the master cylinder, which you did. You're right about the vacuum booster, but I thought I'd throw it out there. You didn't answer if you bench bled the master cylinder. I haven't researched this but is there a rod adjustment from the pedal to the master cylinder? How about the rear brakes adjustments? They're supposed to be self adjusting, but?? I've read a lot of posts about lousy Chevy brakes here and I'm just wondering if your ABS is bad, but once again I'm just throwing that out there?? I really doubt the proportioning valve is at fault. So, bench bleed the master, bleed the wheel cylinders starting with the RR, LR, RF & LF. Manually adjust the rear brakes. Check if there's an adjustment from the pedal to the master?.............Steve

[95TahoeBoston]

Yes, I bench bled the Master.

Anyway, it looks like this may be a failure of the ABS Hydraulic Control Unit, what Chevy calls the "Brake Pressure Modulating Valve Kit" at $556.75

I found a WONDERFUL article about it here:

http://www.babcox.com/editorial/bf/bf50434.htm

The only problem? No junkyard around has the part used. It has to be from a 95 ONLY.

Anybody got a spare EBC310 from a 95 4x4 ?

$600 is out of the question for this vehicle. It's also going to need a speed sensor or two which was turning on the pump which probably hosed the valve to begin with.

So I'm thinking my next best option is to hydraulically bypass the ABS module, but I can't seem to find a proportioning valve... I'm thinking it's inside the ABS unit, since there are wires going into the hydraulic unit, probably to monitor the proportioning valve?

Anyway, like I said before this thing has had squishy brakes since day 1, and I did find one person who said that replacing the proportioning valve took some of the springiness out for him. Also it's worthwhile to note that with 175,000 miles (the other 25,000 is being dragged behind my motorhome) I have NEVER replaced the rear shoes, and they look like only about 50% gone, so this proportioning valve has never been doing a very good job.

I tested the rear brakes and with the drums removed the auto-adjusters worked just fine, so it's not an adjuster issue.

So I guess I want to install a manual proportioning valve in place of the ABS unit. Any clue where I can pick one up, and are they hard to adjust properly?

[95TahoeBoston]

6 months later and my truck is still disabled by this problem.

I can't find the part used, and there are 5 different size lines so re-plumbing doesn't look like something I can tackle.

Any other suggestions?

I shoulda just taken the thing off the road for a year and saved the insurance to pay for the part..

[777stickman]

http://www.gmpartsdirect.com/
I checked this site for price and came up with $381 + $84 s&h=$465.
Part # 12389539. To be safe, e-mail them with your Tahoe VIN to make sure it's the right number for the ABS module. They usually take a couple of days to respond.

Did you ever try removing the ABS fuse and see what happens?........Steve