Help with brake issue

[robm-951]

I have a 68 firebird that's running the CPP big brake kit that uses the Silverado front calipers with the Lincoln rears. I'm using all the factory hard lines up to the factory 4 wheel drum distribution block. I running a 7" dual diaphragm booster with an ACDelco 18M974 (S10) master cylinder with the front port plumbed to the rear brakes.

The issue I have is the pedal seems to have a lot of travel. With the engine off the brake pedal is firm and high, buts sinks quite a bit with the engine on. also when braking while driving, the pedal stroke is a lot and it starts braking then keeps going down. Braking performance seems good for the limited time I've driven it.

I am wondering if I need a larger bore master than the 1". Any advise or things I should look into?