I had the same issue in my '55. It turned out that the residual pressure valve was faulty. That was seen by the ability to pump nearly 100% of the fluid out of the master, then it would refill after letting off the pedal. If you can double pump and get the pedal up to where you want it, this may be your issue. Wilwood makes an inline 10 psi residual pressure valve that works great if you need it.
