The vehicle: 2001 XJ (non-ABS) 140,000 miles

Recently I *thought* my master cylinder went bad. When I started my Cherokee my brake pedal went straight to the floor, there was a "pop" sound/feeling in the pedal and the brake light came on. I drove up the street and unless I mashed the pedal to the floor, it was hard to stop. I replaced the MC, bled the lines, made sure there aren't any leaking lines or cylinders.

One week goes by w/o any issues and this morning when I start it up, the exact same thing happens. I used a new MC (not a reman'd one) so my guess is that the brake booster is crapping out. Again, I'm not losing fluid anywhere and things were normal for 1 week. I'm not saying it's not possible I got a defective new MC, but I have a feeling that's not the issue.

Thanks in advance for any insights.

 

How does the pedal feel with the engine off? If the pedal is hard and it stops the jeep, it's the booster. Disconnect the hose and plug the manifold and drive it until you get another one. It will require mucho pedal pressure to make those discs work though.