89 Chevy Suburban 1/2ton 4WD Brake Bleeding Problems

[Nickoli]

A few weeks back I had to stop pretty hard and blew the main rear brake line at the worst place ever to work on it. I had to limp it home on front brakes and lost all the fluid. In the process of fixing the blowout I replaced almost all of the main rear line, the hydraulic rubber hose, and the two lines to the wheel cylinders due to rust. After all this I was unable to bleed anything from the rears, so I had purchased a brand new master cylinder because I couldn't get any fluid when breaking loose the main line at the master cylinder for the rear brakes either. I thought that maybe the front plunger inside the master cylinder bottomed out and stuck when the brake line blew out and the pedal went to the floor. In the process of working on all this, good ol' Murphy's was here before me and rusted the caliper bleeder screws and I broke them off and one of the rear wheel cylinder bleeders was giving me trouble so I replace both calipers and both wheel cylinders with new re manufactured ones. Now I got frazzled working on this every night after work so I pushed it aside for a couple of weeks. I recently got started on it again and bench bleed the master cylinder per instructions and installed it on the truck and proceeded to follow the manual for bleeding on the truck. I was not able to get any fluid to the rears again and after unsuccessful tries went to the front calipers and could bleed a geyser out of each and the pedal drops to the floor. Went back and tried to bleed the rears again and nothing and the pedal never goes to the floor while bleeding the rears. Now after beating my head several times I crawled back under the truck and broke loose every connection on the combination valve(proportional valve) and retried the process over. This time I was able to get fluid from each of the lines coming from the master cylinder and each of the lines going to the front calipers, however nothing from the main line going to the rear. Now after all this I believe that the combination valve(proportional valve) piston is stuck and blocking the rear side of the valve. I have done some searching and found that if the little needle under the rubber cap sticks out when pressure applied to the pedal and then when released needle retracts that the proportional valve is good. It would appear that I need to recenter the valve in order to get fluid to the rear. I read that some old time mechanics take a large hammer and pound the pedal as hard as they can one time or slam the pedal with foot as hard as you can one time and this will reset the piston in the proportional valve. So far non of this has worked for me. I found that I am not able to buy a replacement combination valve at any of the auto box parts stores and am being pointed towards the dealership for replacement. Somewhere I read an article that said if you purchase a new valve it will come with the piston blocking the fronts so that you can get the rears bleed. If this is the case will I have the same issue bleeding the fronts or after the rears are bleed the valve will center itself? Is there some specialty tool I missing to get the valve to recenter or am I just plain missing something here??? My wife says shes tired of pumping the brake pedal(bun/thy master) and I'm tired of laying on my back dodging all the rust that falls everytime I touch something! I have never had this much trouble bleeding any brakes and any help on this would be grateful. Sorry for the long post but figured the more facts the better!

[Tux_Rules]

There may be other issues here, as from what I read (a quick read at that), the proportioning valve only operates when in a panic situation, otherwise flow of fluid is unrestricted. This is where/what I read:

http://www.babcox.com/editorial/bf/bf50014.htm

The only thing I can think of is to either pop the top off the master cylinder and crack the back bleeders and walk away for 20 minutes or so and try to let it/them gravity bleed or maybe buy a vacuum pump to suck/force the fluid to the back.

EDIT:
Another option is to open both bleeders, put a rubber hose on each one that will go into a pint jar (one on each side) about a quarter full of clean brake fluid. Open the bleeders about a turn, make sure that the hoses are submerged in the fluid, then start pumping the brakes. This will push air out and suck in fluid. After doing this for a little bit, all the air would be pushed out and the line should now contain fluid only. Once fluid is flowing, close one bleeder and bleed the other, then reverse (close that bleeder and open the other).

[DLUVLY]

Bleedin Brakes

Can u post picture of truck I had same problem might be able to help

[MXtras]

Back bleed with a handheld oil pump can and a section of clear tubing. Pump the fluid from the wheels to the master cylinder. If done correctly, your brake lines will not have a single bubble of air and your brakes will feel rock solid.

Read up on the proportional valve. That could be a problem, too.

Scott

[Terry Lingle]

Due to age and water in the brake fluid over the years the proportioning valve has stuck when pushed out of its normal position. You may get it back into center but will it work reliably if it is ever neeeded ? ( say you blow a line )
A new valve will allow you to bleed the brakes and return to center after you finish.

[calweld]

Nahhhhhhh,,,,,, go the cheap way, try to fix the one you have, don't worry about it until the next time you have to stop hard,,,,, how often does that happen, anyway??

[Brianstick]

I had a similar problem years ago on a GM vehicle. I called the dealership and talked to a mechanic over the phone. He said to bleed the brakes in an X pattern, meaning front right, left rear, front left, right rear. I did as he said and my brakes were brought back to normal. Proportional valves hardly ever fail. Hope this helps.

[Nickoli]

Update...No luck or just my luck, I found out that GM has discontinued the proportional valve for this truck as of early last year I did however find what appears to be a direct GM aftermarket replacement on ebay for roughly $49. Of coarse there is always searching the bone yard but I tend to forego the headache and hassle of ending up with a duplicate bad part and out the time and $$$.

I definitely believe I have this problem, When a rear pressure loss occurs then the pressure differential piston moves toward the rear brake circuit and closes off the fluid passages to the rear brakes. When dealing with this type of valve you must re-center the valve before the rear brakes can be bled. Bleed the lines at the master cylinder and the fitting on the valve. Open a front bleeder and spike the brake pedal a couple of times taking care not to exceed half pedal travel. This usually will re-center the piston. If it won't re-center, the combination valve will have to be replaced.

Thanks Tux_Rules : I have not tried opening the front bleeder and spiking the pedal yet due to time and weather. That web page shed alot of light on what I was experiencing and speculated as the problem. After researching the web(Thank you Mr. Al Gore) I found there isn't to much taught about proportional valve 101 these days to mechanics due to a very low failure rate.

Thanks to everyone else's replies as well! A lot of good ideas however almost everyone of them had already been tried and still same result. I will follow up with an update as I work through the problem! And if I could make it home b4 the sun goes down I'll try and snap a pic or two of the truck!


Thanks N!ck

[SMS]

Do you have anti-lock brakes on this truck??

[Nickoli]

Do you have anti-lock brakes on this truck??

No ABS on this truck!

[Slonaker]

I'm pretty sure I have seen this problem addressed before on the board I frequent, but don't remember the solution.

Try this:

http://www.classicperform.com/TechBook/BrakeTroubleshoot.htm#testprop

If that doesn't help, try searching the appropriate year forums here (note that there is a sub area for your suburban near the top of the page).

http://67-72chevytrucks.com/vboard/forumdisplay.php?f=4

Slonaker
'86 C10

http://www.slonaker.net/gallery/displayimage.php?album=3&pos=1

[acwd1950]

Is the brake light on? Or is it on when you pump the brakes? I think the proportion valve is the problem. The front brakes are working and the back ones have no fluid in the line. So the valve thinks that the line is still broke on the rear and in return closing off the rear ports. Heres an idea unhook the line on the master cylinder going to the front and take a line and run it back into the master cylinder and then pump the brakes. It might "trick" the valve and let the thing go the other way, by taking the pressure off the front brakes. Or another idea is to make a line and bypass the valve and bleed the rear brakes then hook it back up and open the front bleeders and hit the brakes and see if that fixes it. Just my 2 cents

Steve