Hello,

It sounds like the piston is stuck inside the proportioning valve.

You will need to disassemble the valve and clean it.

The technique I use is as follows:

Mark the valve body and the mounting bracket, including the direction the bracket faces, so you put it back together correctly.

Loosen the 4 nuts & bolts that hold the halves together.

Put the valve in a vise to hold it together, preferably with soft jaws to protect the factory set adjusting stud.

Do not loosen the jam nut or attempt to adjust the valve!!

The vise has to be at least a 6" vise.

There is a stiff spring inside the valve, but it is not long so it will not shoot out.

Using the vise will allow you to relieve and then reapply spring pressure.

There is a gasket between the halves, but it is only for dust.

Once you have the halves separated, remove the large hex plug from the valve body side.

Then carefully push the piston out.

Clean the valve body and the piston with brake clean or mineral spirits.

There is a little valve on the end of the piston closest to the large hex plug, make sure it is clear.

IIRC, there is a hole in the side of the piston and that should also be clear.

Make sure all the parts are clean and dry before reassembling.

Reassemble with clean brake fluid, preferably DOT4 to lubricate the bore and the seals on the piston before reassembling

If the seals are damaged, I believe there is a repair kit.

But in our experience, if the valve isn't leaking, all it should need is a thorough cleaning.
--
Eric
Hi Performance Automotive Service (formerly OVO or Old Volvos Only)
Torrance, CA 90502