i had the same problems several years ago when i converted drum to disc ( front brakes) on my 69 camaro. i ordered a master cylinder set up with an attached disc/drum residual/proportional ..solved my problems.
i am trying to think of the company..i know they were from California and 55/57 chevys were their main service line..
i will think of it and post if it comes to mind.
__________________
Bjuice..
"I'M YOUR HUCKLEBERRY ! "