where to buy stock brake proportioning valve? Brake problems.

Pedal return comes from a spring inside the booster. leaking vacuum is their most common fault but who knows what other faults might be in a used booster.

Factory service manuals state that proportion valves are not serviceable.
I do recall seeing rebuild kits available for them though.
A dealership might be able to help. I don't know what a NOS part would cost.
Surely more than the aftermarkets adjustable proportion valves.
Only used one I would purchase would be from a donor that still had a clean and closed system ( recently parked ).