It might be a good idea to try to narow down what is bad about the brakes. As noted, earely XJ brakes aren't anything special to begin with, but they should at least stop you. If you jam them on on gravel, do all 4 lock up? Is the pedal firm, or soft? Can we presume that the booster is good, since you replaced it (i.e. with foot on pedal, start engine, pedal sinks the appropriate distance)?
One suspect might be the proportioning valve. If that's clogged, seized or not working right, it could basically be cutting your rears out of the circuit. But I don't know any good way to test that except to test the brakes and see if you can lock the rears.