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?
1992 XJ -- stock (RIP)
2000 XJ -- lots of basic mods. Sold to fellow forum member. I miss it.
2014 JK -- the project is just starting!
For Sale to Good Home---2000 XJ Factory Service Manual (Printed Version) PM for details.