I've been working on my YJ for a few months and and finally got to drive it a few weeks ago. It had some brake problems. I am able to pump the brakes with the engine off and it will get a good pedal and I get nothing but fluid out of the bleeder screws. When I start the engine I loose all the pedal I had plus the pressure does not build. I can open the bleeders and get very little or no fluid being pushed out. I changed the booster today and it helped alittle. I am using a factory YJ booster and MC. I have a new E350 MC that I am going to try tomorrow. There are not that many parts to a brake system but I have not had this problem before. Anyone ran into this.