98 xj 4.0

Car in front of me stopped short so I had to hit the brakes pretty hard but not an all out emergency stomp. Pedal grabbed for a second then popped straight to the floor. There was enough grab to stop me thankfully. Brake light came on immediately. I have some braking power but only at the very bottom of the pedal near the floor. After it happened I checked the reservoir and it was full. Had about a 5 mile drive to my destination. When I got there, front brakes were warm and the rear drums were cool to the touch. Not sure of that means anything but wanted to mention it. Any ideas? I need to diagnose and fix this asap. Thanks in advance!

if the master is still full, crawl under the back of the jeep and take a good long look at the rear brakes, the high pressure may have bursted one of the rear brake cylinders..causing the "shuttle" in the proportioning vale to shift..setting the "brake" light.

look for oil running out from the drum down to the tire..or oil "stripes" on the inside of the rear wheels.

If you don't find any leaks, I'd bet on the seals in the master cylinder being toast.

Similar thing happened to me except my rear metal brake line had rusted through and I lost fluid. Check you metal brake lines front and rear while your at it.

If your master is still full, good chance it's a master cylinder failure

More then likely this is you problem. If a line or a seal had let loose you would have lost the fluid in your master cylinder.

I would agree with that to some extent, execpt the short drive time after the failure, and that fact that all modern cars/trucks run a "dual" master cyl for just this problem...a catostrpic failure in one system or the other will still provide a measure of safety. cars built before some date in the 60's ran a single master cyl, and any failure there resulted in a complete lack of brakes....the same reason semi's run two air systems...and have spring brakes the lock automatically upon air loss.

such a short drive cycle mas have just shifted the shuttle in the prop vale effetiv;y closing the ports to the rear brakes.

UPDATE

Thanks for the replies. I checked today and the reservoir was empty. I looked and found a leak from one of the rear brake lines. If I replace the lines, fill and bleed the system am I good to go or is there more to it? Is replacing the lines a difficult job?

no...just bleed real good, and while your at at it might as whell check the rear brake adjustment.

Thank you. Can you tell me where/how to check the adjustment?

Also is it recommended to upgrade to stainless steel lines?

Thank you sir!

hope it helps takes very little time to do