Proportion Valve on 93 Ford F250 7.3

I would look for a crack....it might not be apparent unless under pressure....

Replace it and bleed the entire system, longest run to shortest.

How's your master and wheel cylinders?

I am not certain about your application, but I have seen some assemblies that

require the bolt threads themselves to be sealed,as well as a gasket,because the

internal threads penetrate into a reservoir or pressure port.

I would also try a new gasket.

The old one may have taken a "compression set"

or hardened, and no longer be functioning properly.

Compare the difference in thickness of a new gasket vs the old one,you will probably

see a difference.

Google a parts diagram for the proportion valve to determine whether this might be

the case in your situation.

Just out curiosity I looked up your master cylinder,and I do not find a separate

proportioning valve,it appears to be built into the master cylinder,or mounted on the firewall separate from the master cylinder.

Are you talking about where the master cylinder attaches to the Vacuum Booster ?

If this is where it is leaking, you need to rebuild the master cylinder itself.

I have seen them leak to the point that the booster was actually full of brake fluid.

A picture would be very helpful,to help us help you.

Don't take chances with your brakes! If the proportioning valve is leaking, change it. The cost is relatively low for the part and it's pretty easy to change.

There are two ways to bleed your brakes. Either use a brake bleeder pump at each brake cylinder or caliper, or manually bleed it. When you bleed your brakes manually, you need to be careful when pushing the brake pedal (if you have an older master cylinder). The reason is that there may be some oxidation on the piston and when you push the pedal to the floor, the oxidation will scratch the seal and you'll have to replace the master cylinder. Put a wood block (or something similar) under the brake pedal when you bleed the brakes.

Good luck and let us know how it turns out.

I don't recall the proportioner being attached as you say. Skwishy brakes that work after a few compressions, yes? Time for to R&R the master cylinder

Dry the area well with brake cleaner, and let it dry. Have an assistant push the pedal gradually while you watch. It could be a brake like with a crack at the flare, or the fitting loose. Or as you suspect it could be the valve itself. If it is the valve toss it. after you get the new one) They were never meant to be repaired. Buy new, or if needed, get a salvage one, but get rid of the leaker. Good luck.