help with 1967 gmc brake issues

[arcticman]

I have a 67 gmc 4x4 drums front and rear. Does this truck have a proportioning vavle? there is a block just off to the side of the master cylinder with electrical wire but no switch/pin to reset valve.The master cylinder has very thick syrup sludge so im replacing the M/C W/C and lines wondering if the block i spoke of can be flushed out or does it need replaced. If it cant be cleaned were should i look for a new one that is correct. my search only comes up with valves for front disk rear drum? Truck has manual brakes no power and im not interested in conversions

Thanks Scott