Does anyone know if I can use a different proportioning valve for my 73S (front discs, rear drums) other than the one from Inline Tube? Last new unit now leaks at the front rubber cover, and I would love another source. Maybe another year or even make? Thanks in advance.

 

I had the same problem, along with some of the fittings constantly leaking. I went with a Wilwood adjustable prop valve from Summit for the rear and ran the fronts straight from the MC.
Obviously it won't be correct from a factory/purist standpoint, but then my car is far from factory anyways.

 

I think just about all GM front disc-rear drum cars from '71 through the '80s used the same proportioning valve, so you should be able to find one fairly easily at a salvage yard. I am sure someone with specific details will post up.