Bad Proportioning Valve?

[jtbartsch]

Hi all, I'm a frequent visitor to this forum when in need of information. Currently in the midst of a rear end rebuild and could use some help. (This will be my first time posting so I'll try to keep this concise.) Upon tear down of the rear end, the rear brake lines were too rusted and broke off when removing rear wheel cylinders. I have since gotten new ones rebent and rerouted from both rear wheel cylinders to the T union above the pumpkin. When I went to bleed the brakes, starting on rear passenger side of course, only a trickle of brake fluid was produced. Found that it could be because of the loss of fluid during the tear down that I had triggered the proportioning valve piston to slide over. I have further assessed this by bleeding the front brake with a helper to try to get it to recenter the piston with no luck after trying to bleed the rear passenger brake again. My test light confirms this as it shows there is ground. (See Picture) I'm thinking that it might be gummed up and having a hard time recentering. I pushed on the pin/button on the left of the unit but to my knowledge this would only work if it had been the front brake lines needing the fluid to be stopped. Currently, I'm at a bit of a loss on what I should do next. Do I buy a new part and risk overseas quality? Do I remove and try to move it manually after removing plastic plug? Is there anything else I can do that I have not already done. Thanks for any insight you all can provide. Truck has front disc brakes & rear drum brakes (13").

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[Ricko1966]

They can be a SOB some times, I can't remember my exact method of making them recenter,I think it was stomp the brake pedal hard a couple times then open the right front bleeder and have someone step on the pedal and watch the light. Try similiar combinations I don't remeber for sure.When the light goes out unscrew the plastic switch and screw in a centering tool,then bleed the brakes. The threads are likely to peel off the plastic and stay in the combination valve. From memory you will need a 1/2 20 tap to clean the threads to screw in the centering tool. Assuming that all happens,leave the centering tool in place after bleeding to keep crap out of the hole,and order a new switch.

 

[Blue Ox]

That valve doesn't shut off the fluid flow. It only works to illuminate the warning light. You'll get it to recenter when you get the system pressure rebalanced.

If you have 13" drums you probably have a LSPV which is attached to a link on the rear axle housing. If it's not too rotted you may be able to crack a line on that fitting and see if you can get some fluid to that point. It's also possible that valve is hung up and blocking flow.

 

[Ricko1966]

On my truck you absolutely cannot bleed the brakes two man style, without a centering tool. It will push off to one side and it absolute cuts fluid flow off. That is one of their intend functions so you don't pump your fluid out if you have a leak on front or rear.

 

[Blue Ox]

They say it, but they don't do it. Both sides of the system are isolated anyway, and I've never had a line fail where the light comes on and it doesn't empty half of the system.

 

[Old Guy Bill]

In the second pic it appears that you’re using a pick but just pushing on the switch itself, that’s not a plastic plug it’s the switch for the brake failure light.
That switch has to come out to use the centering tool, and the tool should be inserted before attempting to bleed the brakes.

 

[Ricko1966]

They say it, but they don't do it. Both sides of the system are isolated anyway, and I've never had a line fail where the light comes on and it doesn't empty half of the system.

Sometimes they do it,I guarantee you my truck will trip and shut off fluid to the rear if you try to bleed the brakes without a centering tool. It's not the only one I've encountered that worked correctly. I guarantee you,mine shutoff the fluid and you cannot bleed the brakes 2 man style without the centering tool. I tried,and tried and tried somemore,because it's a PITA to get that switch out and the centering tool in ,look I had to remove the shroud and it was still a pain because I had to chase the threads. But thats what it took.. Easy peazy after the centering tool was in place.

 

[Old Guy Bill]

I’ve had to f#$& with them before, they are a pain, years of corrosive brake fluid gumming them up inside.
Here’s a link to what’s inside one:

1978 - 1982 GM 1257208 Combination Valve Rebuild | Muscle Car Research LLC

www.musclecarresearch.com

 

[jtbartsch]

They can be a SOB some times, I can't remember my exact method of making them recenter,I think it was stomp the brake pedal hard a couple times then open the right front bleeder and have someone step on the pedal and watch the light. Try similiar combinations I don't remeber for sure.When the light goes out unscrew the plastic switch and screw in a centering tool,then bleed the brakes. The threads are likely to peel off the plastic and stay in the combination valve. From memory you will need a 1/2 20 tap to clean the threads to screw in the centering tool. Assuming that all happens,leave the centering tool in place after bleeding to keep crap out of the hole,and order a new switch.

Do you think if I were to push the brake fluid in the line from the rear it would repressurize and slide over that way? Similar to vacuum pump but other direction(flow towards MC).

 

[Keith Seymore]

No.

K

 

[fast 99]

We ran into a lot of problems with Ford's when these first came out. Valve was not self- centering. We were able to re-center them by opening one side or the other and slowly depressing the pedal. That procedure pushed the valve across. If the brake light flashed, then went back on opened the opposite side. It was a trial-and-error sort of thing sometimes not knowing what side it had slid to. Of course, as they aged crud made centering more frustrating. Really can't remember having problems with GM vehicles. Obviously, they had a similar system.

 

[Ricko1966]

Do you think if I were to push the brake fluid in the line from the rear it would repressurize and slide over that way? Similar to vacuum pump but other direction(flow towards MC).

Get a friend have him stomp on the pedal,have him pump the pedal and then push and hold while you open the right front bleeder. I dint remember the fancy dance steps to get it to pop back to center but it's just messing with the pedal and bleeders to equalize the pressure.if you are handy and in a time crunch you can make a centering tool. Take a 1/2 20 bolt drill a hole in the center and tap in a split pin from the hardware store.

 

[jtbartsch]

Get a friend have him stomp on the pedal,have him pump the pedal and then push and hold while you open the right front bleeder. I dint remember the fancy dance steps to get it to pop back to center but it's just messing with the pedal and bleeders to equalize the pressure.if you are handy and in a time crunch you can make a centering tool. Take a 1/2 20 bolt drill a hole in the center and tap in a split pin from the hardware store.

I have done this about four times. But will keep trying! Thanks so much!