four times in the past five years, the power brakes on my 55 Thunderbird have failed. If I can keep my head, a couple of quick pumps on the pedal will bring them back. The local brake guys can’t figure it out or duplicate it. About 1500 miles a year, no accidents (yet)!
The problem might be with the cup seals inside the master cylinder.
These cup seals are supposed to expand so they seal against the bore as they move forward and apply pressure to the brake fluid. If the cup seals and/or the bore of the master cylinder are worn, they’ll fail to seal properly and no hydraulic pressure will be produced. Sometimes pumping the brake pedal will cause them to expand far enough to where they seal, and hydraulic pressure returns.
Does the problem occur after a lot of hard braking? I ask because moisture can collect in the brake fluid near a wheel cylinder. The moisture boils from heat and you get a steam bubble in the system. After a cool down, everything is OK.
With the exception of the master cylinder, any other brake system failure would result in an external leak that should be visible.