Booster? Check reaction disc inside booster , on the end of booster piston inside engine bay.

If your back brakes lock up first, then you'll need a proportioning valve on the rear line. I just got one for mine, need to fit it yet.

What car did you get your "splitter/proportioning valve" from, for your setup? Stanza one? or original 1200 stuff?

Does mainly sound like air, or stuffed bore in the master cylinder.