I’ve noticed something odd, and have two guesses but want feedback from those who might be better informed.

1992 Corvette loses brake pedal height while on the freeway. I don’t touch the brakes for long periods of time and the pedal is low when I finally get around to using it. One pump and it’s back in action. Stays firm, can’t push it to the floor, no noises or any other symptoms. Brake fluid level is correct.

My guesses are that the master cylinder is going bad, or I have a vacuum assist of some kind that might be failing? Or the vacuum system has a leak and loses suction somehow?

I don’t know how the vacuum assist thing works or even if I have one. I just feel like it’s more likely the problem as the master cylinder malfunctions I’ve experienced have all let the pedal drop to the floor or feel really sponge like.

Any guesses?

Only happens while freeway driving. Never at a stop light or during city driving.