Probably a master cylinder problem
I own a 1949 Chrysler New Yorker with vacu-ease power brakes. Following replacement of the brake shoes, this problem started: When I press on the brakes everything is fine, but sometimes when I let up on the brake and then press the brake again two or three seconds later, the brake pedal goes clear to the floor. If I quickly pump the brake pedal then braking is OK until the next time I press, release and press again within two or three seconds. I have bled the brakes five or six times using a clear plastic bleeding hose and no air is visible coming out of brake bleed screw.
When the brake pedal suddenly goes all the way to the floor and then comes back up after you pump it a few times it usually indicates a faulty master cylinder.