Ok I didn't want to post this here as it should be a very simple problem and if it needs to be moved to newbie go ahead; I just didn't want a bunch of idiots asking if I bled the brakes but it is starting to piss me off and I need my DD back so here it goes: 93 YJ, stock axles for now, 33s, new rotors, pads, drums, good shoes, new MC, all new fluid (flushed out the old stuff), everything has been bled thoroughly and is adjusted up right. When the Jeep is not running I have a great pedal and it holds pressure fine, no sponginess either. As soon as I start the Jeep the pedal drops ALOT, not the inch or so it should, the brakes still work but the pedal has way too much travel before you can feel it working. I pulled the cap off the MC and pushed down the pedal to confirm that it is moving the MC piston by the bubbles and it does move the piston both with the engine running and not running. I know everything is fine mechanically with the brakes, my concern is why the pedal drops so much when the engine is on and why there is so much travel before the brakes have any feel? Is it possible that the booster could have something wrong internally which would cause this? I have only had a bad booster make the pedal too hard and not give any boost, never give too much??? I'm about ready to yank the whole damn system out and swap over to some real brakes on this thing.