W30post

I have a failure on a rear brake system on a GM car that isn't my Oldsmobile that I think is because I had a total failure of pressure to the rear of the car. I replaced the bad wheel cylinder and now the master cylinder but when I try to bleed the system all I get is a very slight drip from the rears. The front is ok.

I think the rear is being checked off by the proportioning valve in fail safe mode. Is there a way to reset this valve or is it shot now and has to be replaced? This is kind of off topic from Oldsmobile but still may help someone else out if an answer is given.

Anybody know how to correct this proportioning valve?

J-(Chicago)

I have this problem as well.....

The valve is bad, and around 150-200 bucks If I remember correctly.

You can boneyard a couple for free though

W30post

J-Chicago,
Have you ever tried to reverse pressure the check valve system to see if you could revive it?

My thought today was to break the line loose at the master and at the rear axle "T" fitting. Then put in 10-20 PSI at the axle "T". I know I would have to have a hose on the line at the master to keep fliud from blowing everywhere but it should not check over on the fronts as they would still be closed off and pressurized.

Just a thought but is it worth the effort to try?

Thanks,
James

J-(Chicago)

I personally haven't tried anything like that, I'm thinking even if you revive it, the pressure distribution will still be incorrect.

J-(Chicago)

The metering valve, pressure differential switch and proportiong valve all are set to spec by the weight of the vehicle.

You want your rear brakes to engage slightly before your front, and if you can't even bleed them, I'd imagine the ratio is WAY off.

The proportioning valve only lets a certain portion of the pressure through to the rear wheels so that the front wheels apply more braking force. So if equal braking was to go to all four wheels the rears should lock up first. If your valve was set at 80%, and the brake pressure was 100 psi your rear brakes would get 80psi

I just boneyarded a few and used the one that worked the best.
The second one I tried worked nice so I kept it on.

Sorry I can't be of more help. Ready your line wrenches soldier.
Perhaps you could engineer a homemade psi valve testing rig, and rent it out to the rest of us when ours go bad

Dapapadon

I haven't had this problem so I can't comment firsthand. But I do have an old 70 Chevy pickup. I visit www.67-72chevytrucks.com and the guys over there have posted about similar problems. You might do a search there & see what you find.

I seem to remember that they vacuum bleed the rears and the system worked.

Don

Jim's rare 80

If I remember correctly, some of these valves have a re-set button on them. Is your brake light still on?

joe_padavano

I have had this problem. First, that's not a "reset button", it's a disable button that must be depressed to properly bleed the brakes. Second, you are correct. The problem is that the valve spool inside the proportioning valve has been pushed all the way to one side and is blocking the rear brake passages. I've been able to move it back by vigorously pumping the brakes. Failing that, loosen a FRONT bleeder screw and press on the pedal. There should be enough pressure in the rear lines to push the spool the other way (frequently TOO far, in my experience ). You might need to go back and forth a couple of times. Be gentle on the pedal.

J-(Chicago)

Interesting. I love learning new stuff. So Is it going to be trial and error back and forth until he finds the right pressure distribution?

W30post

Joe and others,
Thanks for the help on this. First, I think the light is on but it is my father-in-laws car and I really don't remember. I will try late this week to see if I can find the happy medium on getting this valve checked back over. My thought on pressurizing the system from the rear might just be the ticket to this problem. It could push the valve back over to the center while the fronts are sealed. Never mind htat it could force out some ugly fluid also... I think I have to be gentle though or I could damage some older seals somewhere.

So what is this button you are talking about. I don't remember seeing it anywhere, but I wasn't looking either. What years did they start putting on this button?

I was thinking before that I would have to remove the extension hose from the rear and blow in to that hose. I think now I might be able to apply 10-20 PSI at a bleeder with no problems as the entire brake system is intact drums included. The new wheel cylinders should be able to handle 20 PSI easy. Thoughts?

Again thanks for the input...

joe_padavano

The brake system is designed to see hundreds of PSI, but what you propose is way more work than necessary and will reintroduce air into the lines. Just crack a front bleeder screw and step on the brake pedal.

W30post

OK, finally I fixed this problem. It appears that bleeding this system out with a vacuum pump is the way to go. I forced air from the rear to the front with the pressure of an air hose at around 45 PSI while the master as disconnected. Then I immediately followed by vacuum bleeding the system until I got fluid to the rear. Problem solved.

Thanks for the advice.

J-(Chicago)

Glad to hear it.