1968 C10 CST DRUM/DRUM Proportioning Valve Problem

[choctawchevyclassics]

Just purchased a 68 C10. Driving home from work today the rear brakes started locking up and not disengaging. It has DRUM/DRUM set up with power brakes and a proportioning valve. It sounds like the proportioning valve is bad to me. I can't find one online for DRUM/DRUM. Can anybody point me in the right direction?

The rear brakes disintegrated and were locked in the braking position until I released the bleeder screw on the wheel cylinders.

I checked the wheel cylinders and they both move in and out freely, making me think proportioning valve. Any words of wisdom?

I am also going back together with all new brake parts, but can't find the valve anywhere for DRUM/DRUM.

Attached picture shows the assembly in question.