brake problem

[nvrdone]

So here's the story:
I have a'49 3100 that I am trying to convert to power brakes. I installed a braket kit from CPP with the recomended Mustang master cylinder & booster. I also installed the 1/2 " extension to the pedal linkage. I am using the stock pedal with the under cab mounting along with the stock 11" drums front & rear. I did install a 10 # check valve in the m/c since the m/c is for a disc / drum combo.
Now the problem. The pedal goes down about 4" before the brakes engage. Ive got the brakes adjusted to a slight drag, the system has been vaccume bled etc.
I'm at a loss to solve this problem. Any ideas ????
Thanks.