Brake problems

[stone36]

Hey guys, this is a weird one...

I put my foot on the brake and go from park to reverse, I ease off the brake a little and begin to back up(still foot on the brake but only half pressure), when I go to press the brake harder to slow down to go from R to D it doesn't bugde, AT ALL. I can press with both feet and it wont move.

I have to take my foot off the brake entirely and pump once or twice and till will then stop fine.

It only happens in reverse, although it may have to do with each time I start the car since I almost always back out of a parking spot.

Any Ideas????