Brake proportioning valve (located in engine compartment right below the brake master cylinder). Is it rebuild-able? If not, where does one get one, and how much?
I've yet to ever do surgery on a true "proportioning valve (oem-type)" that was doable. There are no service parts for those and never were, they are a safety-related item and should not be scruud with!
Just getting a rotten one apart without destroying it is a total crapshoot and takes much finesse.