brake issues, out of ideas...

So this problem just keeps getting weirder. I bench bled the m/c, which worked. I put it back on the car and leaned on the pedal with good results. As soon as the car moved the pedal dropped a bit and a quick drive around the block showed no signs of improvement. The pedal will still disappear on me now and again. Pumping it a few times brings it back. What gets me is why the pedal changes when the car is moving, there is no vaccum booster and or vacuum equipment on the system.