Strange Brake Problem, 2001 Max

[HawkeyeEngineer]

Hey! I have a question about my 2001 Maxima SE (5 speed manual if that makes any difference).
Note: This ONLY happens on the highway after driving at high speeds for 25 minutes+ and I've had a diagnostic completed by a local auto shop, they claimed the brake system was fine.

The brakes will sink to the floor, but I can pump them back up. I've tried testing the master cylinder and it seems to hold pressure just fine when I'm sitting still or at low speeds. Does anyone have any ideas of what it could be?

Thanks!

Geoff

[Kompresshun]

Geoff, I would say it sounds like you're master cylinder is the problem. That would be my first guess, even though you tested it, there could still be a problem with it. It's also possible that the power brake booster is bad.