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?

[Zspoiler]

I have a 1979 Z-28 RS Camaro Pro-Touring car with a Baer 13" front and 12"inch rear with PBR 2 piston calipers. And a 1982 Corvette master cylinder with a 1980-1 Firebird /Trans Am proportioning valve .and don`t have a problem.Maybe a third generation Camaro/Firebird proportioning will work as well.

[robm-951]

Thanks for the reply. I'll look into that proportioning valve. I was planning to just start with the distribution block and see if it needed a proportioning valve. It also looks like the 82 corvette master cylinder is 1.125 bore as well. I would like to find a master cylinder similar the one I have now but with the larger bore.