I have a 87 z28 came with a 9 bolt disc all the around. I swapped out the rear for a ford 9 inch from quick performance. Still using disc brakes in the rear. I have had this setup for about a year,but fell like i dont have good brakes. I drag race this car more then i drive it on the street and it does have a healthy cam. When i am stagging the car i must press the pedal as hard as i can to the floor engage the line lock,Relase the pedal press the brake pedal down hard again and then relase the line lock to get a good hold. It seems like the this is the only way to keep from pushing through the lights. So im guessing while i have the front brakes locked with the line lock the second pump of the brake pedal is pushing all the fluid to the back brakes. Question is how can i do all this with out pumping and holding, Its like flying a plane at the staging lane,lol.

I was going to upgrade the front brakes but it seems like a pressure problem of the back brakes not getting fluid. Also they have been bleed and have no air. The back brakes are all new lines,rotors,calipers. All cam with the new rear end

 

Have you tried replacing the proportional valve spring I wasn't getting much stopping power to my rear disc brakes when I did a drum to disc brake swap and like many I swaped a 1ncr5 grainger spring in and it stopped very quickly after that.

 

I just was reading about that swap, I thought going from having disk to just swapping for different disc I would be good. The car never had drums, I think I may try the spring upgrade like you just talked about