I was driving to work the other day and I was coming to a stop and my foot went to the floor.had barely if any ability to stop. ended up turning around and going home, I ended up using the e brake the rest of the way, and with luck made it to my garage. so I poked around under the jeep and didnt find a single leak along the lines, fittings, nothing. so I popped the hood to look around the master cylinder and nothing, dry as a bone. I did have a spongy brake pedal before this as well. any ideas?

 

Are you noticing any smoke from the exhaust? Could be the internal seals of the master cylinder; if they go bad what will happen first is that the pedal will slowly sink to the floor while sitting at a stoplight, then it will get worse and worse until you don't have brakes. It may or may not leak out of the master cylinder, but if it does, it will not show up externally but will go into the brake booster and eventually into the engine, where it will make whitish smoke.

 

I was told that the seals could have gone bad but no white smoke. is there a way to just replace the seals or does the entire master cylinder need to be replaced?

 

best way to go new master

 

agreed, unless you are really broke or have a lot of free time, new master is the way to go. Older components are often pitted inside the bores so you end up having to hone them when you rebuild, and half the time they'll be too bad to clean up easily anyway, but you won't know that until you've already bought the seal kit and have the component disassembled on the kitchen table.

 

well brakes arent really a specialty of mine but I dont have the cash to take it to a shop so im tackling this on my own. can anyone explain how to replace the master cylinder?

 

It's really pretty straightforward. Only special instruction would be "bench bleeding" it before bolting the new one in (should be able to find a how to online) and then you have to bleed the brakes afterward. I like using a pressure bleeder, but if you have a helper there's no downside to pump-bleeding a new master cylinder (although using the pressure bleeder can prevent problems when you are bleeding for other reasons, pump-bleeding a used MC can cause the seals to fail.) Only special tool you'll likely need is a set of line wrenches AKA flare nut wrenches. Not sure if the lines on a XJ are metric or SAE, I don't actually have mine yet.