so heres the deal, i replaced the front calipers, resurfaced the rotors and pads, they are on and fit nice. i then replaced the brake line from the master back along with a new flex hose and lines to each drum. upon further inspection i decided to get and install new slave cylinders in each drum along with spring. everything is tight and snug. i bled all four wheels twice last night and when the engine is off, if i pump the brakes a couple of times i get good pressure. when the engine is running i have no pressure at all and i'm pissed. where did i go wrong or what could have failed if there are no leaks?

I suspect you still have air trapped somewhere, do some more bleeding.

Is the vacuum hooked up to your booster? If it is then you still have air in the lines somewhere.

alright, hello again. i went back and did some more bleeding, unfortuneately i'm about to destroy something if i cant get this ****ing thing to work. when the vehicle is off or in the accesory postion i have as much pressure as a brand new car but when i turn the damn thing on !POOF! no ****ing pressure! the vacuum line is in place, all conections are snug...WTF is wrong here?

Where the vacuum line connects to the power booster you should have a check valve its a one way valve , check it . Did you replace the booster or just the MC ??

He says that the booster is new. I think they come with that elbow that has the check valve. At least the NAPA ones do. I still think it is fluid related.

i have the same problem. ive bled the thing so many times and i have no clue. Did you figure it out yet? my next plan is to take the old brake pedal from the cherokee that i got the booster/master cylinder out of and then adjust the booster rod to the length of the old booster.

Did the master cylinder ever run dry? If so you will have to take it off and bench bleed it. One more thing it sound like you pretty much replaced everything from the master back its going to take a lot of bleeding to get the air out. I just did the master on mine and it took 3 1/2 bottles of fluid before I good pedal.

Using junk yard parts for the braking system isn't the safest approach.

ABS or non-ABS?

the master cylinder was leaking so I got all new booster and master cylinder

New to you, or new from auto parts store? Was MC bled properly? Was booster adjusted properly to MC? Certain there are no leaks in newly installed lines? Check, double check, tripple check, then check it again.
Checked for leaks at calis and wheel cyl?
What procedure are you using to bleed?

+1!

i've used junkyard boosters a few times. not to say you got a bad one.

stoopid question, but did you put the wheel cylinders on the correct sides?

Did you bleed them in the correct order? I can't seem to find the post now, but there was another recent thread that had the bleed order listed. Apparently if you don't do it in the correct order, you can end up just moving air around the system.