I am going to go out on a limb on this one - I had a similar problem, but not in a Jeep but in a car with just hydraulic brakes, no power assist. The brakes seemed to bleed out OK, but then went weak and mushy. Turned out it was sediment in the brake fluid reservoir that had plugged up the small hole that leads into the master cylinder - so they bleed out OK, but when the piston retracts, then there is no fluid or very little that enters the cylinder...just a guess on my part. But, get 'em fixed and by an expert if you need to - don't mess with safety.
Richard <img src="/forums/images/graemlins/cheers.gif" alt="" />