My proportioning valve will not let fluid out for the rear brakes. I have replaced everything in the braking system with the exception of the hard lines which are in good shape and clear, confirmed this with brake clean and an air compressor blow out. I removed the proportioning valve from the car and I cant clear what ever the obstruction is for the rear brakes even tried a piece of stainess welding wire to poke it through and nothing dead stop.
Is the proportioning valve rebuildable? OR can you get a direct replacement? This is for a 73 Road Runner
Thanks,
Jeff

This company has many brake valve kits. They also have online diagrams.

https://www.musclecarresearch.com/

They suggest using a hydraulic grease gun for blowing out blockage in some cases.

I had the same problem with mine couldnt no fluid to the back brakes what so ever. It was my proportioning valve i ended up buyin a new one. Take it apart and try soakin in that barrymans carb chem dip cleaner, let sit in there for a few days. I still had no luck after doin that.your better off buyin a new one tho. If you get new one theres companys like inline tube,classic industries,year one etc that will have them.etc

Should come apart easy enough. Can you post picture?

just take a wrench to undo the top, and wrench to undo the big bottom bolt. its real easy to take apart and put back together.
https://www.musclecarresearch.com/1970-76-mopar-combo-valve-autopsy

Thanks for the replies. This is frustrating. Here are some pictures

That combination valve has a slider in it to stop fluid flow to front or rear in the event of fluid/pressure loss in either channel. There isn't much to them. I've since removed mine in favor of an adj. proportioning valve, but rebuilt the one I have. Remove the switch and the reset pin and the shuttle valve inside that is currently blocking your rear channel (I had to use compressed air to remove mine). Clean everything up, and if the seals are in good shape, you can probably re-use. I took the whole thing to a hydraulic shop and we ID'ed and replaced everything except (if I recall correctly) a teflon that was in good shape anyway and the rubber boot around the spring loaded reset pin.

I soaked everything but the electric switch in carb dip for a couple of days, then cleaned as best I could. I used crocus cloth on the shuttle and in the bore. In the bore I used a gun patch style needle eye rod in an air drill with a strip of crocus cloth through the eye. I cleaned the switch with electrical de-greaser and that was good enough.

It has been a while, but that is the general gist. I don't recall it being a hassle, but the reset pin has a spring under it and maybe a couple of washers, so watch the small parts.

All that said, you may be able to just pull out the switch and reset pin assembly and flush it with brake cleaner to get the shuttle sliding again. After the flush, use some brake fluid as lube and be careful as you don't want to score the bore or shuttle up.

Even simpler and maybe effective, just push in the reset pin and see if the shuttle will center up.

There are a bunch of more detailed threads on here regarding that valve, and it would be a good idea to read all of them. Most of the advice is to replace it with an adjustable proportioning valve. I think that is the best guidance unless you have no performance aspirations for your car. With the adjustable prop valve, you can reduce rear braking force and avoid rear brake lockup when you jump on the brakes. The shuttle valve/warning indicator is a pretty good safety feature though, as you won't lose all your breaking in the event of a leak and you'll get a warning light which will be of dubious value because you'll notice something is wrong with the brakes as the light comes on.

Just so you know the brass chevy valve, PV2 or PV4 are nearly a bolt in swap. Only one adapter is needed if keeping your stock lines. You can either remake the rear brake feed line from master to the PV or use an Adapter in the PV to accept the stock line fitting size. You can grind off the bracket from your old valve and reuse it to mount the new valve on to the inner fender stock position.

Interesting. AC DELCO 172-1353 (disc-drum) and 172-1361 (disc-disc)

Here is the exploded view of your style prop valve.

Great Info Thank you!!! I really want to keep the original style PV in the car. The car is a survivorish type of car ( is that even a word?) granted not a high dollar car BUT want try to keep it original as possible.

Great thread! I have learned a lot here. Is this worthy of an archive or is there already one like this there?

Here is photo of brass valve installed. Master cylinder and booster are out right now so easy to see.

Grind off rivets on original valve to remove the bracket.

If you eliminate that valve for an adjustable valve you will loose your "redundancy" in the brake system. Kinda voids the fact you have a dual master.