My buddy has an 89 xj and keeps having problems with the brakes. Seems as though air is somehow getting into the system. Has changed master cylinder and all rubber brake lines. Has bled the system several times. It will be fine for a few days and then the pedal wants to go all the way to the floor and hardly stop. Any ideas?

 

Is the fluid level dropping in the reservoir. Has he checked the hard line for any kinks or binds

 

Fluid doesn't drop at all. Hard lines all seem to be fine.

 

Does he have the Bendix 9 ABS? You can tell by looking- that system has a pair of high pressure vessels instead of a big round vacuum booster.
That system is a nightmare to bleed and get working right. See post #19 in particular here:
https://www.cherokeeforum.com/f2/ben...-14960/index2/