When you press the brake pedal and hold it, and it gradually sinks down toward the floor even though you are not moving, that is a sign of internal hydraulic failure in a worn master cylinder. The master cylinder is the heart of your braking system. It converts the mechanical pressure from your foot into hydraulic pressure that activates the brakes on each wheel. If its internal seals are failing, fluid bypasses them under sustained pressure, causing the pedal to slowly sink. And when the brake pedal hits the floor, you lose all braking power!
Here is the fuller picture of what might be happening
A brake pedal that sinks at a stop but comes back firm when you pump it is a classic symptom of a master cylinder with worn internal seals. The seals can no longer hold steady pressure; they hold briefly when you first press, but they leak internally under sustained load. This is different from an external brake fluid leak, where you would likely see fluid under the vehicle or notice the reservoir getting low.
That said, an external brake fluid leak can also cause a sinking pedal if enough fluid escapes quickly. Check your brake fluid reservoir under the hood. If the level is noticeably low, there is a leak somewhere in the system: brake lines, calipers, wheel cylinders if your Jeep has rear drum brakes, or the master cylinder itself.
Either way, a brake pedal that sinks to the floor is a serious safety concern. You can experience dramatically reduced stopping ability, especially in an emergency hard-stop situation. This is not a drive-and-see situation!
If your Jeep's brake pedal is doing this, we would encourage you to contact Hoover Street Auto Repair in Ann Arbor right away: this problem warrants prompt attention. See our brake inspection service or information about our general brake repair.