I have a 1994 Cherokee 2wd country withe the 4.0l. I purchased it after it had been sitting for a couple years. Didn't take much to get it running but had no brakes. I've replaced everything with the brake system except the Proportioning Valve and the lines going to the calipers and wheel cylinders. The issue now is when the vehicle is off i have great pressure on the brake pedal, but when you crank it up, it goes to the floor. I believe it is a vaccum issue because the engine idles a little low (around 400-500 rpms) and runs a little lean, when you unhook the vaccum line from the brake booster to the engine, it starts running much better, (around 700 rpms) and not lean anymore. There is still some braking power when you get to the floor, its not enough to lock the brakes up but they will stop it. Naturally I through it was the the brake booster or could be some stubborn air in the MC, i then replaced the new MC and brake booster again in case it was bad parts but they its the same with them. ive bled it every which a way you can multiple times, bench bled the MC, had a friend press the pedal and bleed them, gravity bled them, vaccum bled them with a hand pump and an air compressor adapter, all multiple times. Im kind of at a loss to what to do to fix the issue, any help would be greatly appreciated.

 

Classic bad brake booster. It's leaking engine vacuum internally. That's why your idle improves when you unplug it.

Even though you replaced it, you could have gotten another faulty unit. The diaphragm inside is probably torn. Get a better quality booster from a parts store and try again. The symptoms point directly to it.