Hey everybody. I've had my Cherokee XJ 4.0 1989 for about a year now, and I've basically not changed anything (except headlight switch).
Yesterday i started my car to go to work, and i had almost no brakes. It was early in the AM and seeing as I've noticed that it has been doing this thing where it doesn't really build pressure before running for a few minutes, i thought nothing of it. (Stupid, i know). I knew something wasn't right, but i haven't had the time to check it out yet.

Problem is, it didn't return pressure like it normally does, instead it simply makes the classic whoosh sound and goes all the way to the bottom. Now, it feels like it's trying to build pressure, but it doesn't really work. Instead i press it to the bottom, and then it kind of has this little click, and goes just a few millimeters further down.
At the same time as it clicks, the brake light comes on. When this happens, it's like it can magically build pressure again, but even though it builds pressure, there is still no braking power before i almost reach the bottom. pump the brake a few times, and the light deactivates again, and no pressure again.

The fluid was pretty much empty (2-3 millimeters left), so naturally i filled it up. Didn't help.

Now i haven't had the time to do anything to it yet, but was hoping that somebody might have had the same problem (i know) and figured it out.

Thanks a bunch.

 

Sounds like it is leaking fluid somewhere (calipers or rear cylinders) and eventually emptied the reservoir to the point where air entered the system. Could be other things, but based on you finding the reservoir nearly empty, that is a strong sign that fluid is escaping the system somewhere.