Brake pressure drop while backing

[tree98]

Try driving down the same hill loaded going forward instead of backward. It could be that the direction of travel has nothing to do with it. Could be that the hill your backing down is steeper than most your driving forward down and your on the brakes a lot harder than just normal driving????

Sounds like a bad air leak somewhere to me, maybe a brake chamber, valve, etc. Chock your wheels, turn the key on(engine not running)release the parking brake and push the brake pedal down. A leak as severe as your describing won't be hard to hear.

If you still believe it's only while in reverse, put it in reverse and see if you hear anything.