no brake pressure

I am experiencing a loss of brake pressure when my xj is running. I adjusted and bled them(disc/drum combo) and but still have pedal bottoming out. it feels good when the car isn't running, but I lose them when it starts up. Is this a vacuum problem?

If I'm reading you right and the pedal is high when it's off, and sinks when you start, it sounds like the opposite of a vacuum problem. When the car isn't running, the pedal is firm, but there's no boost. A system that is leaking down or in need of bleeding may feel good when there's no boost. As soon as you get boost, it pushes harder, and whatever is failing shows up. If this is what happens, then your booster is good, and your vacuum is good.

You're going to have to do a little more diagnosis to get better answers, though, because just bottoming out could be many things.

Let's start with what happens when you pump the pedal up, with boost. Does it firm up, or stay down? If it firms up, you might need bleeding or have radically unadjusted rear drums. If it stays down, then something else is the matter, possibly a failed master cylinder, or a mechanical failure in the rear brake that goes beyond mere bad adjustment.

If you pump it up and it seems firm, does it sink slowly under pressure? If it sinks slowly, it's the master cylinder. Often a failing master cylinder will hold at least for a while under a sharp push but sink under gentle pressure.

I assume that if you've bled the system you have also checked your fluid levels and scanned for leaks, so my bet is for either a really bad rear adjustment problem or a bad master cylinder.

While you're at it, it would help to try taking the vehicle on to a nice loose dirt road or driveway, and try locking up the brakes (assuming you have enough brake to drive cautiously to where such a place can be found). Check to see which, if any, wheels have locked up. A failure in one circuit of a dual master cylinder will cause the pedal to go almost to the floor, but then it will engage the un-failed portion at the bottom. So, for example, if you find a little bit of brake at the bottom, and it locks the fronts, you know the problem is in the rear brakes or the rear portion of the MC.

Finally, if the problem appears intermittently while driving, occasionally a soft pedal, and then it recovers, check for bad wheel bearings on the front.

Thanks for response. I adjusted my rears out until they drug on the drum, then backed of a notch. I bled each one out, starting at the farthest drum, then went back to front.Each time it would get a little better but the pedal would eventually fade to the floor. From what I read in your response, this points to a bad M/C.

I'll pick one up and try it. How should I prime it before I install?

Follow up. Jacked up the back and the rears grabbed, but at the bottom of the peddle

Sounds as if it's the MC all right, especially if it fades to the floor and there's no external leak. Do check very carefully for leaks, since a bad caliper seal could do this too, without losing a lot of fluid visibly from the reservoir, but assuming no leaks, go for the MC.

There are various ways to bench bleed the MC, starting with just filling it on the workbench, pumping it and keeping it full until fluid consistently comes out the outlets, or installing it and pumping with the brake lines loose and letting the fluid dribble out as you keep the reservoir full. Actually, it's probably a good idea to do both these things. A less messy method is to get a kit (not sure where or what price, but I've seen them out there, and have read that some new MC's even come with one) with temporary lines that plug into the outlets and loop back to the reservoir, so you just pump the fluid through without having it dribble all over. That also allows you to push more fluid through without worrying about how much you're wasting. It can take many strokes to bench bleed fully, so be patient.

You'll still probably need to bleed the rest of the system when this is done, but with luck you'll be good to go after that.