Chevrolet Repair Questions? Ask a Mechanic for Answers ASAP
Hey this is MATT. Wow, it sounds like you have tried everything!
Okay, right now, your pedal still goes to the floor? Try pinching the rear lines again and see if you have a good pedal? (like you did before). If so , then the problem has to be beyond the point that is clamped. Does the e-brake hold? Let me know, and I'll try to help you figure it out. MATT
The E-brake is currently disconnected and when I pinch off the back brakes the pedal does still come back. When I raised the back wheels off the ground, I started the truck and put it in gear. The rear brakes will stop the wheels, but I have to push the pedal nearly to the floor.
Sorry to repeat, but you are saying that you do have a good pedal with the rear hoses clamped? Correct?
I have to say, I'm a little stumped. But If that is the case, it must be something in the rear. Try unclamping one side at a time, and see which side makes a difference? MATT
Yeah, i understand. I wasn't sure if you had separate hoses in the rear or not. The only thing I can think of is when the front pads were replaced, the calipers were compressed ( without opening bleeders), and fluid ( and contamination) was pushed back up into the abs system.
Did the dealer do an ABS modulator Bleed, using there TECH II scanner? I'm not sure why clamping the rear, solves the low pedal?
I've been searching the ITAN (International Auto Technicians Network) for answers. So far, the best I've found is the dealer tech II bleed procedure?
But you have replaced the rear WC's, and have no leaks in the rear! ??
I'll keep looking. Can you find out if the dealer did the ABS bleed? I wouldn't assume that they did, Ask to be sure. MATT
Okay, i thought you may have changed the pads first, then did the calipers. The sucking air part has me puzzled also? But does make me think that the ABS could be involved. This is really weird since you did not push fluid back up into the lines during the repair. Mechanics do that all the time, just compress the piston, and push the fluid back into the lines out the M.C. Usually never causes any problems! BUT!!
It is known that contamination builds up inside the caliper, and then it can cause problems when pushed back thru the lines into the ABS sys.
Was the E brake disconnected before all of this work? And why? CAn't say it is related. Just trying to think of everything?
My best Idea, is the dealer ABS BPMV (brake pressure modulator valve) bleed procedure. If I can come up with anything else , I'll let you know. MATT
The combination valve is connected to the BPMV, which is connected to the EBCM ( elect. brake ctrl. mod.) . I haven't had any luck finding a pic for you. But I would ask if they used the TECH II to bleed the ABS system.
I'll keep searching for you, MATT
Well, I found plenty off problems with a low brake pedal after replacing front pads. But no definite fixes. From what I found.... everyone said use dealer front pads, and bleed the ABS sys. I'm not sure why the pads would make a difference?
What gets me is that it seems to be the rear thats causing the low pedal?
From what I've found, it is a somewhat common problem with no known fix. Hard to believe. I'll keep looking. Sorry , I don't have the answer for you. But I have not given up. Let me know if you have any thing to add.
I will keep looking, but if you want, I can relist your question. MATT
where did you get the wheel cylinders? I've talked to people that had problems with after market WC sucking air. They used GM WC's and all is good.
I've come across two cases. Some pinch off the front and have a good pedal, and some the rear. In the case where pinching the rear gave a good pedal, it was bad WC's. I can only tell you what I've heard? But from what you have said, it sounds like the rear is where the problem is. And Faulty WC's is the only thing I can think of, which has also happened to others. MATT
The problem came about before I replaced the wheel cylinders. The old ones were not leaking. I only changed them because I didn't know what else to do. I wouldn't think that both sets of wheel cylinders would be faulty.
Yes , I agree with you. Just passing along any info that I have found. Like I said, It seems to be a common problem with out a fix!
Some people clamp off the front and have a good pedal, and some the rear. I've been told to use OE front pads. I personally can't see how a different pad can fix a low pedal?
How about this, Are the front slides in good order? When you first install the new pads and calipers, I grab the caliper and make sure it moves. Slide it back and forth, making sure that it is free. If it sticks it can cause a low pedal. But this doesn't explain why clamping the rear gives you a good pedal?
Have you driven much? Too see if anything changes, or you get an ABS light?
If I come up with anything else I'll let you know. Good luck, I'm trying MATT
Try this test for the modulator, can't hurt. MATT
ABS clinic / we discussed this problem. Your problem may be the ABS modulator (especially if the calipers were collapsed
"old school" without opening the bleeder & clamping the flex hose) here is there procedure for diagnosing the modulator
1)locate modulator assy. (large ABS component with multiple lines)and remove the small rubber caps (2 or 4 depending on
2)inspect for brake fluid, if fluid is present replace modulator.
3)straighten out a paper clip and insert into hole under rubber cap until it bottoms.
4)Feel the paper clip while someone applies the brakes in a NORMAL manner with the engine idling. repeat 2 or 3 times
for all holes. any movement of the paper clip movement means brake fluid is bypassing the normally closed ABS dump
valve and requires modulator replacement.
NOTE: The modulator section is available separately from the computer section. I hope this helps mike. Good luck, Troy-------------------------------------------------------------------------------------------------------------------------------------------
Low pedal problems.....use OEM pads, slides free?, abs techII bleed, test modulator (above), OEM WC's, warped rotors or bad bearings
(pushing pads back in)
I will try to test it tomorrow and get back with you then.
Wow, I'm sorry if I'm letting you down. Take a look at this pic I found, does it look like what you have?
Also the following is some info I saved from IATN. Some of it, I have already said. But it explains how complicated the bleed procedure can be. MATT
(pushing pads back in when wheels turn/ causing low pedal) --------------------------------------------------------------------------------------------------------------------------------------------
Another thing to check to see if the modulator assy is junk is for bypassing fluid from the accumulator portion of it. To
check the 2 accumulators you must remove the rubber caps(2) located below the combination valve portion. (easy to
remove, hard to put back on). You take a skinny screwdriver with a shaft 5-6' long and carefully insert it into each hole. It
will bottom out. Hold it lightly and have someone depress the brake pedal. If your screwdriver pushes out at your hand on
either one, the accumulator portion is shot and the modulator assy needs replacing.
Also look to see if fluid leaks out the valve stem area on the combination valve. It has a rubber cap too, but you can see
the stem once you remove it. It should not leak.
The ECBMS (brake computers) I'm told have a fairly high failure rate with the relays in them. You need to have new GM
hold down screws available if you change ECBM's as the old screws tend to seize in the modulator body and snap off. New
ones get torqued to specs.
At the point you are knee deep into the hydraulics end of things and replace the modulator assy (the big chunk with the L
shaped brake computer on top of it) you MUST replace the transfer tubes. The transfer tubes can only be gotten from GM
and are part of the combination valve that bolts on the side of the modulator assy. The combo valve assy. has the 2 lines
from the master cylinder going into it. It also has that valve stem that separates the front from rear systems like all the
combo valves before it and the (red) brake lite switch. Torque specs for the 3 allen head bolts are first 6'lbs, then 16'lbs.
Bleeding it requires the patience of everyone. If you even suspect the master cylinder of possibly pooping out during the
1/2 gallon or so of DOT 3 you will use if you only have to do one 3 stage cycle, replace it and avoid having to do the bleed
cycle numerous times. The master cylinder has to be bench bleed prior to installation.
The 15 second or so wait between pumps of the brake pedal doing the manual type bleeding we did almost puts you to
sleep, and this is why they give you about 2 hrs in the book. You still get to do rr,lr,rf & lf in that order. So here we go....
Make sure the master cylinder is full.They wanted you to have a clear hose attached to the bleeder and put into a clean
jar of DOT 3, but our bleeders didn't have enough end to hold onto a plastic hose, so we just caught the fluid as it
streamed out, and closed the bleeder quickly.
Here is the weird part. AllData says you have to depress the combo valve stem the entire time you bleed the system. Yes
you do. We didn't have the GM special tool yet (J-23709 Kent Moore) so a friend came over to babysit the valve stem and
hold it flush with the nut that retains it with a plastic screwdriver end. (don't push it in, or accidentally bend the valve stem
if it protrudes either)The two man job is now a trio.
This manual bleeding is not like conventional non abs brakes and can only be done as follows. Try not to let master cyl get
below 1/2 full.
Helper is in cab. Person at wheel bleeder first opens bleeder, then helper in drivers seat pushes brake pedal down gently
about 1/2 way. Close the bleeder screw, and everyone waits 15-20 seconds. The wait allows the fluid to refill the m/c
piston area completely before reapplying the pedal. Repeat until you don't have any air. This may take up to a pint of fluid
per wheel, no exaggeration here at all! (don't jam that brake pedal to the floor or you run the chance of trashing the
master cylinder seals.) Done at the rr? Good, now on to the lr and so on.
Now you get to do the fun part. Automatic Bleeding. Still making sure the little stem doesn't poke it's head out at you, you
hook up your scanner to the OBDII connector and get to the autobleed menu. During the Autobleed, Mastertech scanner
has you apply the brake pedal, and then firmly apply the regular brake pedal as the scanner takes over bleeding the air
out of the modulator block. Don't open any lines when autobleeding, and keep that little valve stem in it's place.
Lance, Glenn and the GM info say with the newer systems you only have to autobleed cycle once. On older systems
sometimes you had to run autobleed as many as 3-4 times to purge the modulator block. You have to remove your foot
from the brake pedal between cycles. The brake pedal rises and falls as the test cycle takes place.
Just when it was getting to be too much fun, it's back to the manual bleeding cycle again. Be patient again as it takes a
real long time with just that little stream each pedal pump to get air from the engine compartment out to the bleeder you
are working on, and you most likely will get air. Our valve stem, master cylinder filler (yea you have to keep the master
filled and not let it run down too far on juice) helper noticed a change in the sound during pedal pumps as we gradually
got the air out of the system. It went from a gurgling whoosh to almost no sound at all.
When done doing bleeding, check the pedal for height and firmness. Go out and do a bunch of regular and ABS stops and
see if the pedal action stays acceptable, or if it sinks or warning lights go on. If your pedal stays firm about 1/2 way
through it's travel, and is pretty much fully applied at halfway you're at about where our new truck was that we used for
comparison. Got warning lights flashing on and off? Check them out. Is the pedal spongy and sinks? GM says keep bleeding
till you get an acceptable pedal action.
For GM's the Gas engines use the Auto Bleed sequence once, and Diesels need to run the Function Test 4 times, releasing
the brake pedal each test.
I went to the junkyard and bought an ABS module, brake computer, and combination valve and put them on. So far this seems to have fixed the problem.
Thanks, XXXXX XXXXX response. If you can, let me know how it goes. I wish I could have helped you better. It seems it's a fairly common problem with no apparent fix.
But I glad you got it, Makes me wonder which part did the trick? The combination valve? Take care, MATT
Yeah, I kinda wonder myself. I don't really think it was the computer because of some of the symptoms and there wasn't a light on or a code. I'm not sure how much the new parts would have costed. Someone told me about $900. I pulled them off a 96 suburban and paid $80. Once I got the part off I saw the two rubber grommets and port holes you had spoken of for testing for failure. Just couldn't find them with part on truck. I'm glad you didn't give up on me when the dealership mechanic did.
Thanks, XXXXX XXXXX like I let you down. But I wasn't for a lack of trying. I agree with you about the ABS module, you would have had a code and ABS light on most likely. And your probably correct about the cost, you got a great deal. Just save your module just in case.
Also glad to hear you found the holes, I was wondering what was going on there.