proportioning valve trouble i think? on 1990 Chevrolet G Series Van (G30)

OK here goes, 1988 chev camper, 3500 and replaced leaking master cylinder, bleed system over and over and cant get hard peddel and no real pressure at rear drum bleeder when helper pushing peddal to floor? Should have all the air gone and looking at perportioning valve.The little pin moves in and out? any one have a idea?

Asked by for the 1990 Chevrolet G Series Van (G30)
Hold that pin in while helper pumps the brakes!!! PUSH on the pin hard, wont hurt it. That should get the fluid pressure back to the rear so you can then bleed again!! It's designed that way. If you still have a poor pedal, post results for more info.