Disc brake proportioning valve blocked

[mikepasquale1]

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?

 

[Qcode351mach]

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..

 

[mikepasquale1]

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.

 

[Qcode351mach]

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 wont 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.

 

[Tnfastbk]

I took mine off and am not using it. My brakes are doing just fine, but I am running 4 wheel disc also

 

[72HCODE]

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.

 

[mikepasquale1]

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.

 

[rob73stang]

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 ....

 

[mikepasquale1]

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.

 

[Justin71]

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

 

[72HCODE]

you might want to buy a reproduction prop valve for 71-73 just so you can move forward and leave the current prop valve on the bench.

there is definitely something wrong with the safety system as it is stuck to the drum side cutting off the brake fluid.

it could be stuck on a o-ring inside or jammed in a ridge.

i would take the valve apart again and look into the bore see if you spot something, or a torn o-ring or a seal that has expanded from contamination.

 

[wwhite72]

SSBC makes a very nice replacement. perfect fit. plug-n-play. $90 bucks last time i looked.

 

[Justin71]

Does anybody have a diagram that shows this safety system, and how it is plumbed between the two chambers? Actually a diagram of the whole block would be good.

 

[jeff8877]

Does anybody have a diagram that shows this safety system, and how it is plumbed between the two chambers? Actually a diagram of the whole block would be good.

Here is the rebuild instructions with pics...pretty easy to do.

https://www.musclecarresearch.com/valve-rebuild-kh

 

[Justin71]

Yeah, I used that kit and those instructions. Cleaned everything - I even soaked it in ammonia as recommended. No idea what I could have done wrong. What I'd like at this point is to see some detail on the "safety system" and how it's connected to the other chamber (assuming it is). Assuming there is some sort of safety interconnect (makes sense to me), mine must somehow be broken. At this point I'd just like to be able to figure out what it is that is wrong with this unit because when I rebuilt it, all parts seemed pristine (after cleaning).

 

[rocket366]

The layout:

http://www.ebay.com/itm/65-66-68-70-73-MUSTANG-DISC-BRAKE-PROPORTIONING-VALVE-/291478995634?fits=Model%3AMustang

With someone pushing the brake pedal crack the rear outlet line from combo valve and see if fluid comes out. If not

then the combo valve piston is not centered or blocked.

I wonder if the hard line to rear brakes is smashed or the rubber hose at axle is collapsed?

(assuming the master cylinder was bench bled and is giving rear brake fluid pressure)

Keep us posted

Paul

 

[Justin71]

Patience and persistence wins the race (you'll be considered patient if you can get to the end of this stupid long post without losing interest).

We figured it out last night. I found an image online that gave me a clue about the safety feature:



(warning - I'm pretty sure this is NOT an exact diagram for a 71-73 model, but it is I think close - this one doesn't show a spring on the prop valve lower left like we have, and it *might* show a spring in the shuttle valve that ours doesn't have - the unclean chevys have that feature - if you have a spring there - you'll only get an uncentered position -and hence a dash indicator light- if you have a failed cylinder AND the pedal is pushed)

I think the "safety feature" is an interconnect between the shuttle valve cylinder and the proportioning valve cylinder similar to what is shown on this diagram.

Given that thought, here is what I decided to try:

I had the thing in my vice setup like this:



I had the front brake chamber plugged with a bolt as you can see. I had the rear brake chamber live and filled with fluid. The line from MC to dist block was in place and snugged up. I applied hydraulic pressure in the same manner you bench bleed your MC. In the config shown I couldn't get any flow thru to the back brake output at all - no matter how hard I tried.

So, hoping to clear the safety feature, I took the shuttle valve completely out (the shuttle valve is underneath that brass nut on the top of the distro block in the picture). I noted that the shuttle valve piston was inserted pretty far... probably past center. Using needle nose pliers I pulled the shuttle valve piston out. Then I tried again to push fluid from MC into the block. After some pushes, the shuttle valve cylinder filled with fluid - this convinced me they are indeed interconnected.... how else can I get fluid in the shuttle valve cylinder with my setup?

So, I decided I needed to close the shuttle valve chamber back off so I can build pressure. I put the cap on and snugged that down, but left the piston out. This sealed up the shuttle valve chamber. This allowed me to fill the block completely (I capped the forward brake ports too) and get pressure everywhere inside except the rear brake outlet.

With that done, I pushed a few times and got some resistance, then, bam - fluid shot out the rear brake outlet port. Something in that port, or in the prop valve/chamber was plugged up, and i had finally gotten enough pressure in the right direction to clear it out.

From there, I topped off both MC chambers with fresh fluid, removed the shuttle valve cylinder cap and put the shuttle valve piston back in "just enough" to get the cap on, installed the front brake chamber line between MC and distribution block, and sealed up the MC lid. To put it in the car, I was able to work the assembled MC + dist block (with fluid) around behind the export brace and get both the distro block and MC onto their bolt holes. Two nuts on the MC and 2 on the dist block thru the sidewall of the engine compartment and it was in place.

I connected up the rest of the brake lines (being very patient with the 1/32 to 1/16th wrench turns allowed by the tight quarters in this space, all while interlacing my arms around the hood hinge.)

From there we bled the lines, RR, LR, RF, LF in that order.. Getting tons of air out of the RR line before fluid. Low and behold the brake light indicator on the dash went out (right after we first applied some pedal pressure) and never came back on. After the full bleed we had solid pedal, and, for the first time in a long time I think, the rear brakes on this car are working again.

Long road, and it was one of the messiest jobs I have done in a while... At one point I pushed the MC out of the vice and spilled all that fluid, and during brake bleed, I decided I needed to spill the jar I was collecting fluid in too. But all in all - it was worth the effort to get everything working right again. And now I have confidence in the system - which is really important.

Thank so much for all the help everyone. I don't think I could get there (or I don't know how long it might take) if I didn't have this resource to lean on!

Jay