I have a 1967 D600 with a hydro-vac (or maybe a hy-power) brake system. When I let up on the brake pedal it does not want to return unless I pull it up with my foot. I also have the pedal go to the floor from time to time however it has pressure on the next pump. I am going to replace the master cylinder to start with. Are there any trucks to diagnosing and repairing a system like this? Thanks for looking!