Did you check the master cylinder to make sure it didn't or doesn't have a residual pressure valve in one of the ports
if that M.C was for a production car with 4 wheel disc brakes it won't, or shouldn't have one, if it was for a drum brake disc brake car it will have one for the drum side to keep around 10 lbs. residual pressure to prevent the brake shoes from forcing all the fluid back up the brake line into the M.C.