I had issues with my stock brake master cylinder in my 47 Kaiser so i decided to change it out to a dual resivor drum to drum 1969 chevy truck master cylinder. i mounted it on the firewall and made all new brake lines. I bleed them with a mighty vac and the old fashion way with a buddy. The problem is they will pump up and hold pressure just fine but after you let off for a few seconds they drop to the floor and have to pump them up again. My question is do i need to run a residule pressure valve or something?