Probably a master cylinder problem

Question:

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.

Answer:

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.