Booster/vacuum issues have no effect on basic braking, other than making pedal pressures for a given brake force lower.
Since you have NO brakes, you can dismiss them as an issue.
Let's start with the basics---is the master cylinder full of GOOD brake fluid? Have you tried to BLEED any of the brakes, and if so, do they bleed normally?
I don't own an XJ any longer, but I still think they are the best of all the Jeep products ever made. My XJ was my favorite vehicle in my 50+ years of driving.
So...I stick around and give advice.