Drag car brake/proportioning question

The residual pressure valves are there because the master cylinder is frame mounted. This was a common problem with street rods where the master cylinder was low mounted. IIRC a 2 lb valve was used for the rear brakes & 1/2 lb was used on the front along with a proportioning valve. It's been a while since I've worked on that type of system. I had a street rod with Mustang II manual front disc brakes & drum rear brakes with just a proportioning valve that "seemed" to work great under normal braking. The master cylinder did not have the residual valves in it. The first time I had to get on the brakes hard, the pedal went to the floor. Two quick pumps & they were back up. I did some research & talked to some other street rod owners/builders. I put the residual valves on my car & all was good with the world.