1972 K20 Proportioning Valve / Master Cylinder back pressure

[RedRoad]

All new brakes (booser, MC, rotors, calipers, pads, drums, shoes, WC, springs) and lines with the exception of the proportioning valve. Problem is the master has a lot of back pressure causing it to leak on the alternator. Thought the MC was bad and put a new AC Delco unit and the problem persists. Made sure the rod has the proper clearance, bench bled the MC and vacuum bled the system several times. Changed the MC seal, cover and bails and still leaks. When I pop the cover off the MC it sounds like Im opening a can of beer, WTF???

Brakes work well but dont like brake fluid leaking anywhere on the truck. Looks to be a one year only part as well and IT and other line manufacturers do not make the hard line kit for it either due to line routing. I dont think the valves are rebuild able but since its the only thing I didnt change I figure its the issue.

Anyone know how to test, clean/rebuilt or found a source for these units? Visually the 71 and 72 1/2 ton units do not look the same and have the typical hard line routing which is different than a 3/4 ton.

Any info appreciated.

Thanks