Do you have a return spring on your brake pedal, to pull it back from the master cylinder? With your foot off the brake the actuating rod that goes from the brake pedal to the master cylinder should have a bit of free play between the end of the rod and the master cylinder valve plunger, just a millimeter or less is good.

Is your master cylinder under the floor, putting it lower than the caliper/wheel cylinders? If yes, do you have brake pressure residual valves (link - https://www.speedwaymotors.com/Wilwo...xoCJK4QAvD_BwE ) installed, a 2 pound for the discs and a 10 pound for the drums?

Just a couple of thoughts that popped into my head.