My brakes are really weak and I know my booster has a vacuum leak so I need to address that.

Do the proportion valves in the engine bay and near the fuel tank ever fail and do they need rebuilding?

There's a paragraph in the FSM about how to test operation.  Replace if bad.  But how to take them apart and clean them, and what's been found inside, has been described here before.  

Thanks I'll search.

Stick these words in the search box and some good threads will show up - "proportioning valve rebuild".

Parts are NLA, but they do fill up with gunk and that will affect there operation.

I pulled mine apart and cleaned it in this thread. It should give you an idea what to expect.

 

Thanks thats great info