Soft brake pedal / weak brakes at idle

[autumnwalker]

I was using the JK to winch out stumps last weekend and I noticed that the brake pedal was soft and the winch was pulling me into the stump. I was hard on the brakes, at idle, and the brakes were slipping - the Jeep was inching / rolling towards the stump.

I added some throttle to get the RPM up to give the winch more power and I noticed that the brakes also locked the wheels solid and I was then being dragged towards the stump with wheels locked and tearing the grass up.

This was repeatable. At idle the brakes couldn't provide enough clamping force to prevent the wheels from being turned when the winch was pulling me toward the stump. Increase RPMs and the wheels would lock solid and I would be dragged, with wheels not turning towards the stump.

I was thinking bad booster, but that should make the pedal firm. Bad master cylinder?