When climbing, you would not be using the brakes, therefore it should not affect anything. Brakes fades when the fluids are not in good condition and repetitive use of the brakes causes the fluids to heat up resulting in brake fade.
With vehicle stationary, start engine and depress and hold the brake pedal. If it starts to sink after a few seconds, the brake master cylinder is leaking pressure internall and needs to be repaired or replaced.

Answered by KHLow2008 (expert)
40,318 answers provided

Appreciate the suggestion. Tried it. It didn't go down. I hadn't tried this one, this time, but am familiar w/ the concept & used in other situations. This one didn't feel as if it would work & didn't. While not a pro, I did qualify as a mechanic, for the Post Office & have many years under the hood & driving. This is the 1st time I have EVER encountered this & will have to find a solution, b-4 my wife experiences it. She'd panic & might wreck.
The ONLY time it does this is after a steep climb & gas is released. Then, if you press brake, it will floorboard. W/in 30 sec. to a minute, pressure is restored. Haven't tried pressing brake while STILL accelerating. That's next.

Response from Mack
7 questions asked

Try bleeding the system to see if it works. There could be air in system,.

Answered by KHLow2008 (expert)
40,318 answers provided