Greetings.
For the past couple of months i have been trying to get to the root cause of the brakes not working on my '89 XJ, 2.1 TD model.
Among the many things i have done in an effort to fix it, i had changed the Master Cylinder (to no effect) and the Vacuum Pump (which seemed to have done the trick and also solved the problem with the AC actuators, until i tried to bleed the brakes and that was when the problem came back).
I dont think it is the brake booster since despite the hissing sound whenever i press the brake pedal, the pedal remains super soft.
Another thing i have noticed is that in the master cylinder, there is bubbling coming from the hole in the rear compartment (closest to the firewall) though i have no idea where the air would be coming in from.
I have read stuff about bleeding the Master cylinder, but i have no idea how to do that.
Anyone have any ideas?

 

Did you bench bleed the master before the install, and verify the push rod was the correct length. You should also clean and bleed the proportioning valve.

 

Again, i don't really know how to do that unless it consists in opening the cover and just pumping the brakes. In that case i tried, but either there is a MASSIVE amount of air in the system somewhere, or it's being sucked in because there is consistent bubbling from the hole in the rear reservoir inside it when i pump the brakes.

 

It's pretty simple, there are discussions on this forum on the proportioning valve. Check all lines and rear wheel cylinders for fluid, if air is coming in fluid will be coming out as well.