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Disc brake proportioning valve blocked
#1
Im getting fluid at the master cylinder and to the block but nothing to the rear drums. Front discs work fine. Cant push fluid through the block to the rear brakes.

Blockage? Bad seals, o-rings, etc?

I believe this is the corrent rebuild kit for my brake block
https://www.musclecarresearch.com/valve-kit-kh


anyone ever attempt this?

"Do it once, do it right!"
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#2
mikepasquale1;62763 Wrote:Im getting fluid at the master cylinder and to the block but nothing to the rear drums. Front discs work fine. Cant push fluid through the block to the rear brakes.

Blockage? Bad seals, o-rings, etc?

I believe this is the corrent rebuild kit for my brake block
https://www.musclecarresearch.com/valve-kit-kh


anyone ever attempt this?

I just went through this exact same problem 2 weeks ago when I swapped out the rear in the camaro..What it turned out being was the proportioning valve safety feature. Most of these valves have a safety feature in that when one of the circuits (front or rear ) is damaged..has a faulty component..or is open to air..it blocks off that circuit so that you can maintain the good circuit and not lose your brakes completely. There is usually a little pin on the block that has to be reset or pushed in to restore the bad circuit. When bleeding the brakes that pin is supposed to be held or locked in place till the bleeding is done..

LOVE OF BEAUTY IS TASTE..THE CREATION OF BEAUTY IS ART
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#3
I have heard this same thing. There is no reset on this unit. I have heard a way to dislodge it is to open both.front bleeders and slam.the pedal.down really hard but that didnt work at all.

"Do it once, do it right!"
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#4
mikepasquale1;62793 Wrote:I have heard this same thing. There is no reset on this unit. I have heard a way to dislodge it is to open both.front bleeders and slam.the pedal.down really hard but that didnt work at all.
Post up a pic of the valve..Try opening one front & one rear at the same time..Let gravity bleed..Keep the master filled...That should equalize the pressure & let the valve snap back to center..(Worked for me)
Or here's a cut & paste method..

The valve (and light) senses just ONE THING - are the pressures in both halves of the dual master cylinder the same?
A dual master cylinder is two masters in one. One half creates pressure for the front wheels and the other half creates pressure for the rear wheels. Both of these pressure are fed to EACH END of the spool valve that is shown in your illustration. The spool valve is free to move in its bore. The only thing keeping it centered is having EQUAL PRESSURE on both ends of the valve.

If at any time one half the system looses pressure (any leak internal or external) then the pressure from the other half of the M/C forces the valve to one end or the other. When the spool valve is moved it pushes the switch and grounds the light circuit.
Once the leak is repaired THE VALVE NEEDS TO BE RE-CENTERED!
As long as the valve is not centered the light wil stay on.
Nothing will bring the valve BACK to center except a procedure to re-center it. (described below)

Note - GM systems commonly had springs which also kept the spool vale centered. On these systems when there was a leak the light would come on ONLY when the pedal was pressed (and a pressure differntial existed) then when the foot came off the pedal the springs would re-center the valve and the light would go out.

So how do you fix it?
YOU MUST HAVE AN ASSISTANT TO HELP BLEED THE BRAKES. There have been many discussions here about one man bleeding, but this situation IS NOT one of them. You NEED the presssure of a good foot on the pedal. :thup:

1. Fill the master cylinder
2. Have you assistant ready to operate the pedal.
3. Start at one end (front or back)
4. Have the key turned ON so that the light is functioning
5. Step on the pedal
6. Open a bleeder
7. CLOSE THE BLEEDER SUDDENLY MID STREAM! Dont wait until the stream slows and stops!
This process is NOT trying to get the air out. (I assume all the air is out, all you are doing is centering the valve)

8. Did the light go out AT ANY TIME during this process? (remember the key is on. Your assistant should be watching the light)
9 Repeat the bleeding process at the other end (front or back) Open the bleeder and shut it off mid stream under pressure.
10. Did the light go out at ANY TIME during THIS bleeding?

If the light stayed on for both bleeds, the spool valve is STUCK. Time to replace it. (yeah it can be a pain)
As you open and suddenly close a bleeder (under good hard pedal pressure) you are forcing the valve to shift to one side. Since you dont know if the valve is shifted toward a leak from the front or back you need to simulate a leak (pressure loss) on one side or the other.

If the light went out MOMENTARILY (it won't stay out at this point) then the valve is free to move. :woohoo:
Now think back did the light go out when you were bleeding hte front or back? Because that is where your last 'leak' was. The valve shifted to show a 'leak' where you last bled.

11. Now go to the other end and open and suddenly close a bleeder under pressure. This will cause a 'leak' that will shift the valve and by closing the bleeder suddenly the return of EQUAL PRESSURES should keep the valve centered and the light should stay out until another failure occurs.

I have had difficult systems where I have had to go back and forth between the front and back several times until I got the valve centered. Sometimes it requires a little less pedal pressure during the bleeding but you will need high pressure to get a stuck valve moving.

LOVE OF BEAUTY IS TASTE..THE CREATION OF BEAUTY IS ART
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#5
I took mine off and am not using it. My brakes are doing just fine, but I am running 4 wheel disc also
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#6
i had this happen to me, there is a mushroom seal on the prop valve and the rubber basically fell apart and it jammed the prop valve from opening allowing brake fluid into the rear lines.

you can rebuild the prop valve more now then when this happened to me about 5 years ago. back then i ended up mixing parts from 2 other prop valves to make one good one.

you can buy a rebuild kit now for the o-rings and seals. it comes with directions.

before really getting into the prop valve i would make sure the master cylinder is pushing fluid from both chambers. make sure the rear brake line has not collapsed. the rubber line in the rear that goes over the rear axle can collapse internally and stop the flow of fluid to the rear drums.


also you can make bleeding the brakes easy with a vacuum bleeder. If you have issues with the prop valve centering easily with a normal brake bleed from longest line to shortest then the prop valve needs to be replaced or rebuilt because the valve is hanging up or sticking.

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#7
I finally got a chance to get it off of the car. I tore it apart on the rear brake side, enought grit and dirt came out of it to surface a baseball field. I know you can rebuild these but i wasnt too happy with the shape that the piston was in and the fact that i cut my bracket off a long time ago so i bought another used one from fastbackstack through ebay.

"Do it once, do it right!"
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#8
local ford dealer has a centering tool , to center valve and hold it while bleeding .. been there done that 25 year ford senior master tech ....
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#9
I actually made one. I took an old brake pressure light switch, cut the top of it off and drove a screw down through the center of it to hold the pin from being spring loaded anymore. It worked great.

"Do it once, do it right!"
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#10
Guys- I am having the same problem, I recently rebuilt the prop valve, and since that time, I have nothing at the rear drum brakes. I currently have both the MC and distribution block off the car (including the two steel lines that connect them).

Is there a way to bench test the prop valve? I have rechecked everything in the dist block, bench bled the MC, and then I block off the front chamber at the MC wall and leave only the back chamber active. I hook the other line (feeds back brakes) to the proper port on the prop valve, and when I do this I cannot get ANY fluid thru the prop valve. If I remove the prop valve, fluid comes thru easily. Bench bleed and my test shows rear chamber pumps fluid just fine. But when I install the prop valve (yes I have double checked seal directions etc)… no matter how hard I push on the MC, NO FLUID will flow thru the prop valve to the rear brake line. Not a drop.

I can't figure out why it won't flow anything. I know the spring holding the prop valve piston will need to be compressed by movement of the piston due to pressure, but after moving a little, it should allow fluid to flow to the back. No amount of pressure I can generate at the MC (using typical bench bleed method) will open the prop valve enough to let fluid flow.

Anyone have any ideas on what to check/how to proceed? I can prove all the flow paths are open, I've checked installation and seal position on the prop valve, but I can't get prop valve to move under test (with spring installed).
I need some new ideas/insight.

Thanks, Jay

Pics of The Car
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