I had a similar brake problem on my '52 Olds custom It is subframed with an '87 Buick Regal sub, disk brakes. The rear is a late Chev drum. Seven inch dual chamber booster, Corvette master.
The problem was solved with an electric vacuum pump. The brakes on the Olds now react very much the same as on my late model Mercury Gran Marquis. Just light foot pressure is required. I still do not lock up the brakes...it just stops!!!