proportioning valve/combo valve woes

[1low4x4]

85 c20.

Had a red brake light on in the dash for months. Thought it was something to do with a faulty e brake (even rho the e brake worked). finally asked about the light and was told it could be an indication of a more serious problem with my hydraulic system.apparently there is a proportioning valve or also called a combination valve that, when one brake system fails, the valve shut it off allowing you to still use the other system. In my case the rear brakes were completely dry, so the proportioning valve block them off and I've been using front brakes only for 6 months or better.

when the valve trips it also sets of that warning light,but I did not know any of this until after I replaced the master cylinder and the booster. I bench bled the master cylinder before I installed it, filled it up, bled the rear brakes 4 or 5 times, and had fluid but absolutely no pressure. Front brakes have plenty of pressure. So I saw on a diagram as well as researching online that the proportioning valve has a push button that can reset the valve when it trips. Its in an unfortunate spot right next to the frame so I use a clamp 2 reset it, not knowing that its just a small 1/8" thick pin that you push in, or so I think. Well that did not work, so I tried the method of opening the front valves for the front brakes, leaving the rear brakes closed to build pressure up in them, and punching the brakes and that was supposed to reset the valve. That did not work. Also tried going in forward and reverse punching breaks both times to reset. Did not work. Also removed amd cleaned the rear proportioning valve and still nothing.

I decided to remove the proportioning valve, and had to remove the fan shroud to get to it. Managed to get all of the brake lines of without breaking them, and then undid the electrical connector and pulled to both bolts out that hold the proportioning valve to the crossmember. I set the proportioning valve on end and used a small punch and small hammer to try to drive the reset button back in. The small pin was already pushed in, and I ended up driving the entire center section of the proportioning valve in about a quarter inch. After I realize that the only part I was supposed to drive in was the small pin, I gave up and decided to order a new valve. You cannot get them anywhere but online.


does anyone have experience trying to reset these things, have you had any issues with them gumming up inside and not resetting easily? Did I miss the damn thing up by punching the hole thing in one quarter of an inch?

 

[bucket]

Mine have always reset on their own once pressure was built up in the system again. Only issue I've ever had is the light being stuck on despite having all 4 brakes working properly, and the fix for that was just unplug the wire. I don't need an idiot light to tell me when the brake pedal feels squishy, my foot can do that.

 

[1low4x4]

Yeah. I had no luck that way. I guess I'm gonna order a new one tomorrow. Hopefully I get the right one

 

[RetroC10Sport]

Raylanr and i are going through the same problem with his C20. We probably need a new prop valve.

 

[chengny]

iI have always been able to get them to reset themselves.

First you need to repair the leaking hydraulic circuit.

Second make sure your M/C chambers are full.

Then go out in the AM and at the first stop sign/light whatever, SLAM the brakes on.

That will usually center the piston and save you the hassle of diving under the truck.


The only time I ever failed to get results with this method was the last time I lost a brake line. After repairing the line, I filled up the M/C and went to do my usual stomp reset.

Well, let me tell you boy, I stomped the living Shiite Muslims out of the those brakes - over and over - and the light would not go out.

My daughter was watching me do this - and looking at me as if I was nuts - so I began to explain to her what I was attempting to do. "There are 2 reasons that cars will show a "BRAKE" light on the dash. The first reason ", I started off, "is when you have left your e-brake on"

The words were not even out of my mouth - when it hit me - my e-brake was still on. It had been on from earlier when I had the truck up on jackstands.

I reached in and pulled the release - BAM, no more brake warning light!

I don't mean to imply that you are nearly as senile as I am but it is worth mentioning.

BTW - I don't believe there is any externally activated "reset" button. The spool piece that shuttles left or right when no pressure exists on one side of it, is only re centered by re-establishing a tight hydraulic system in whichever side lost pressure and pumping the brakes up (or slamming them on). That "button" you hit with your center punch is just a protective boot. If you really can't hydraulically get the shuttle valve to recenter, the onp;y thing you can try is to pull the body of the switch(white cap in the middle) and use a sharp object to push the shuttle one way or another.

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[1low4x4]

Well yep then I ****** it up.

I can't see anywhere that brake fluid couldve gone. I pulled both rear wheels and drums, cylinders are bone dry. All lines leading up to the proportioning valve are dry. Front calipers are dry. Proportioning valve was dry and all the way up to the mc was dry. Mc was dry on outside...pulled the booster which had been replaced a year ago and no fluid in it, rubber gasket was dry and intact between mc and booster.

I just can't see where that fluid in the rear reservoir couldve gone.

The e brake was the first thing I tried. Months ago lol

 

[chengny]

I can't see anywhere that brake fluid couldve gone. I pulled both rear wheels and drums, cylinders are bone dry. All lines leading up to the proportioning valve are dry. Front calipers are dry. Proportioning valve was dry and all the way up to the mc was dry. Mc was dry on outside...pulled the booster which had been replaced a year ago and no fluid in it, rubber gasket was dry and intact between mc and booster.

I just can't see where that fluid in the rear reservoir couldve gone.


This is new to me - did you post about losing massive amounts from the rear circuit on another thread maybe?

Anyway, more details - that fluid is not evaporating into thin air- we can figure it out.

How much did you lose?

Any repairs before or after the loss of fluid?

How clean is your truck underneath?

Did you trace the common line to the rears all the way along the rail?

What kind of surface do you park on (asphalt, dirt, concrete)?

Do you have any enemies?

 

[1low4x4]

It all started when the light came on. Didnt notice the pedel was squishy until later. Thought it was an issue with the e brake.

Asked about the light the other day. Some one told me to check my fluid so I did. Front reservoir was full but rear was bone dry.

Tried bleeding rear brakes and got nothinh the first 4 times. Finally fluid but no pressure. Followed lines all the way from mc to every wheel, all through the frame.

Only thing I can figure is it leaked out and set off the light 5 months ago, then since then it has dried up from wherever it leaked out.

Under side of the truck is pretty clean, no mud but maybe just thin layer of road grime like all 28 year old vehicles driven daily.

No enemies far as I know.

I park on concrete at night but company gravel daily. Never saw spot on my driveway other than the trans leak from the rear seal.

I just ordered a new combo valve. Plus the tool that allows you to bleed without tripping it. Hooefully I cwn I stall the thing, bleed em, and then just watch for where the leak could be. Only thing I know to do

 

[chengny]

So, you have never even identified a leak?

Only thing I can figure is it leaked out and set off the light 5 months ago, then since then it has dried up from wherever it leaked out.

Agreed, and I have strong suspicion that when you receive your new valve (and finally get the rear circuit to develop pressure) the leak will reappear - with a vengance. You should be able to find it in a hurry.

Also removed amd cleaned the rear proportioning valve and still nothing.

I think you are refering to the "height sensitive proprtioning valve" above. Don't be surprised if - when you get rear pressure - this is found to be the source of the leak. GM issued a TSB in 1990 advising that this component be removed rather than serviced. The stated reason was to eliminate rear brake sensitivity but they had a tendency to leak too. If that is where you find you are losing the fluid, come back - there is a service procedure that explains the disable/bypass procedure.

The various valves are explained below:

These vehicles use either a two function or three function combination valve. Some vehicles use a height sensitive proportioning valve in addition to the combination valve.


The two function combination valve consists of a metering valve and a brake failure warning switch. The hydraulic brake lines are routed through this valve to the wheel cylinders or calipers. The metering portion of this valve assists in providing balanced front to rear braking by delaying full hydraulic fluid pressure to the front disc brakes until the rear drum brakes overcome return spring tension and the linings contact the drums. The brake failure warning switch portion of the valve activates the brake warning lamp when there is a loss of pressure in either the front or the rear braking system.

The three function combination valve consists of a metering valve, a brake failure warning switch, and a proportioning valve. The metering valve and brake failure warning switch operate the same as those in the two function combination valve previously described.
The proportioning section of the valve proportions outlet pressure to the rear brakes after a certain rear input pressure has been reached, preventing rear wheel lock-up.


The height sensitive proportioning valve provides ideal brake balance according to weight at rear axle. This valve is mounted on the frame and responds to changes in vehicle trim height in relation to rear axle load. Mechanical linkage connects the valve to a bracket attached to the rear axle.

 

[friscobob]

If you have power brakes you can be losing brake fluid from the master cylinder into the booster.

 

[chengny]

Mc was dry on outside...pulled the booster which had been replaced a year ago and no fluid in it, rubber gasket was dry and intact between mc and booster.

Apparently he did a good job checking that whole area.

 

[SlickGTP]

The one thing I haven't seen mentioned is wheel cylinders... Have you checked under the drums to see if they're leaking? Sometimes there's enough dust in there to soak up the fluid so that a leak isn't readily apparent.

 

[1low4x4]

Pulled both drums and checked them, one of the first things I did. Nothing. Doesnt mean they couldn't have leaked six months ago and dried up

 

[1low4x4]

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[1low4x4]

Well, installed the new prop valve, got lines snug. Bled them like usual, ps rear, d rear, ps front, dr front.

Got no pressure and very little air out the back. Front wss same lots of pressure.

Pulled the mc cap and my brother accidentally pushed the brake and some air came out of the new MC, even tho we bench bled it.

After that, brake light went off, pressure went through the roof in rear. Lots of air out.

Filled mc bavk up, bled all 4 at least twice. Whole can of fluid through the mc.

Pedal still squishy. Goes to floor if you press hard and long enough.

Took th rear prop valve loose from axle and moved it all around while bleeding brakes though the front feed and rear feed lines that feed and exit that valve. No air. Lots of pressure.

Thought maybe mc was bad? Even rhough its new? Took the old mc and started to bench bleed it to reinstall it and noticed it leaking from the cylinder where the booster pushes through it. Thinking that's where my fluid originally went???

At this point, tthe brake light is off, I have mean pressure at the back where before I didn't, but pedal is still soft??

What should I do?