1989 GMC Sierra....brake issues

UPDATE ON MY BRAKE SITUATION......

I purchased a $30 vacuum pump tool from AutoZone today and followed the instructions for bleeding the brakes. I did the back brakes only, since that is where my problem is. The front brakes work great, and if it werent for them, I would have NO brakes at all. Once I pulled a decent vacuum from the bleeder screw on one of the back wheels, I loosened the bleeder screw and watched/listened as lots of air and little fluid came out. I repeated this procedure "literally" 21 times and got the exact same result each time. There is no way in hell there is that much air in just the rear brake lines. So......I went back to the AutoZone and purchased a set of "one man bleeder screws". Installed them and bled the rear brakes one at a time, making sure the fluid level never got empty. I pumped the brake (truck off) 5 times, refilled fluid and pumped another 5 times. I repeated on the other rear wheel. There was massive amounts of fluid on my garage floor when I was all done. I even hooked that vacuum pump up to the bleeder screw on the dump valve and bled it quite a few times. Satisfied by this point that I HAD to be done, I started the truck and depressed the brake pedal. :banghead: I wasted my f**king time and money. The front brakes still work great and grab well, but it is the rear brakes that have absolutely no pressure. If I pump the brakes with the truck off, the brake pedal will loose half of its pressure, until the front brakes grab.

There are absolutely no leaks anywhere, not the lines, master cylinder or wheel cylinders. The brake booster and check valve have been replaced within the last two years and I have also checked for vacuum leaks, which there are none. Where is the brake pressure for my rear wheels going and why? PLEASEEEEEEEEE HELPPPPPP

Well, I just went out this morning and purchased a new master cylinder. I bled all four wheels again til there was no air coming out of any of them. Started the truck :banghead: Still the same crap. I did notice however that the dump valve keeps filling with brake fluid from the rear brake lines. Every time I pull a vacuum on the dump valve bleeder screw, the rear brake fluid level drops and more fluid keeps coming out of the dump valve.

Is this suppose to happen?

Ok.....I just got done bypassing the entire RWAL. The rear brakes now run straight from the master cylinder to the rear wheels. I connected the two lines via a strong peice of brake hose and clamped both ends tightly....no air leak. I left the front brake line connected to the ABS system and capped off the two open holes where the rear lines were....no air leak detected. I also re-bled the rear and front brakes. Again....same crap. Start the truck and the brake pedal hits the floor.

WHAT THE HELL IS GOING ON!!!!!!!!

Ok.....I just got done bypassing the entire RWAL. The rear brakes now run straight from the master cylinder to the rear wheels. I connected the two lines via a strong peice of brake hose and clamped both ends tightly....no air leak. I left the front brake line connected to the ABS system and capped off the two open holes where the rear lines were....no air leak detected. I also re-bled the rear and front brakes. Again....same crap. Start the truck and the brake pedal hits the floor.

WHAT THE HELL IS GOING ON!!!!!!!!

Did you bypass the combination valve? I suspect that combination valve maybe stuck not allowing fluid to get to the rears. Also, the rear brakes much be adjusted so there is drag. If you spin the rears by hand and you dont hear or feel drag, the rears are too loose and need adjusting.

What state are you in?

//2000CAYukon

Dallas, TX........I just now got done TOTALLY bypassing the entire ABS system. The combo valve and the dump valve. Both the front and rear brake lines now go from the master cylinder to the wheels. I now have half a brake pedal while the truck is running, which is alot better than I had, but it is still the rear brakes/brake lines that arent pressurizing. I continue to bleed the rear brakes and continue to hear a small amount of air blow out of them, but I have ran "literally" 64 fl oz of brand new brake fluid thru the rear brake lines alone. I can find no evidence of any fluid leakage at the wheels or the lines.

Do I have to take the bulb out of the dash so the "brake" light dont stay on, now that I have bypassed the ABS?

Dallas, TX........I just now got done TOTALLY bypassing the entire ABS system. The combo valve and the dump valve. Both the front and rear brake lines now go from the master cylinder to the wheels. I now have half a brake pedal while the truck is running, which is alot better than I had, but it is still the rear brakes/brake lines that arent pressurizing. I continue to bleed the rear brakes and continue to hear a small amount of air blow out of them, but I have ran "literally" 64 fl oz of brand new brake fluid thru the rear brake lines alone. I can find no evidence of any fluid leakage at the wheels or the lines.

Do I have to take the bulb out of the dash so the "brake" light dont stay on, now that I have bypassed the ABS?

Is it your plan to stay with ABS bypassed? You will probably have issues with the rear locking up.

I would tighten up the rear brakes by adjusting them manually. You need to take the wheels off then the drum. Adjust the brakes a couple of clicks. Then put the drum back on. Do this until it is fairly tight and you feel a lot of drag.

//2000CAYukon

I would like nothing more than to have everything as it should be, but the price is getting out of control for my budget. The whole reason I got rid of the combo valve is it was leaking fluid past it into the dump valve. The price for the combo valve is way outside my budget, so I will more than likely have to deal with no ABS at all.

Is the combo valve rebuildable.....easily by myself?
Would the lack of rear drag cause air build up in the rear lines when trying to bleed?

Now comes a difficult question.....

Does anyone know the thread size and fitting size to the ends of the brakes lines? I need the size for the line that leaves the dump valve and goes to the rear brakes, the size that comes up from the front brakes (just above where it coils), and both sizes that enter the proportioning/combo valve.

I am trying to purchase a union that I can connect the front to the front and the rear to the rear brake lines with the ABS system bypassed (taken out). By my using hose and hose clamps for the connections, it looks and feels unsafe.

Also, many thanx to 2000CAYukon for all the assistance.

I would like nothing more than to have everything as it should be, but the price is getting out of control for my budget. The whole reason I got rid of the combo valve is it was leaking fluid past it into the dump valve. The price for the combo valve is way outside my budget, so I will more than likely have to deal with no ABS at all.

Is the combo valve rebuildable.....easily by myself?

First, the combo valve is not really used for ABS. It's job is to apply some pressure first to rears then to the front. It does not seem to be rebuildable in that I can't find parts for rebuilding it. Getting one from an auto wrecker may be another option for you.

Would the lack of rear drag cause air build up in the rear lines when trying to bleed?
The lack of rear adjustment will affect pedal movement and feel. If you are vacuum bleeding, it should not affect anything. If you are manual bleeding it will make it harder to get the air out.

//2000CAYukon

Now comes a difficult question.....

Does anyone know the thread size and fitting size to the ends of the brakes lines? I need the size for the line that leaves the dump valve and goes to the rear brakes, the size that comes up from the front brakes (just above where it coils), and both sizes that enter the proportioning/combo valve.

I am trying to purchase a union that I can connect the front to the front and the rear to the rear brake lines with the ABS system bypassed (taken out). By my using hose and hose clamps for the connections, it looks and feels unsafe.

Also, many thanx to 2000CAYukon for all the assistance.

Back when I worked in a part store, we carried many different size brake lines and fittings. I would take the combo valve out and try to match up the fittings at a parts store.

Using a hose and hose clamps is probably not a good idea on a brake line.

//2000CAYukon

I went out and got some metal fittings to use in place of my hose and clamps. I put them on and they work perfectly, with no leaks at all. I also purchased another wheel cylinder for the rear, which means both rear sides are brand new. I re-bled all four wheels with 32 fl oz of brand new brake fluid. Started the truck......bam, pedal goes back to the floor again. I am at witts end on this. Everything is new and not leaking a thing. The brake lines go from the mater cylinder straight to the wheels, front and back. The master cylinder is new as of yesterday. I have never spent so much money trying to fix brakes. I have even adjusted the parking brake cable and made sure the rear wheel brakes were tight against the drum. I have also made sure the check valve on the booster is not leaking air, and it isnt. As soon as you turn the truck off, the brakes will get stiff but still sink down 3/4 the way to the floor. Even if you pump them up with the truck off, the pedal will still sink 3/4 the way to the floor. Something somewhere is loosing pressure.

What more can be done or can I check?

I was following this thread closely in hopes to solve my exact problem in my 80 GMC.I've done everything from bleed the master cylinder, changed the proportioning valve, replaced all the brake lines front and rear, rear brake cylinders and about 9 huge bottles of brake fluid and nothing.oh yeah..new shoes on the rear as well.out of desperation and peace of mind. I removed the rear brake line from the master cylinder and places a cut nail in the line and reconnected it and presto bango I had brakes only to the front ofcourse lol and NO I'm not running it on the streets that way.I basically gave up on it for awhile and scratch my head everytime I go into my garage and see my truck resting peaceful and have this uncontrolable urge to kick every panel I can reach, but wisdom kicks into play. phew. Let me know if you figure it out and what it was

Just thought I would let you guys know that I sold my truck. My new toy is a 1983 Camaro Berlinetta. It's a 5spd with the V6 motor....ughhhhh, I know, but anyway.....I am sure I will be posting many a many questions about it to come, but obviously in a different room/forum. thanx for all your help guys.