Jeep Cherokee XJ Forum

Nice link for the power brake bleeder. Not a bad price but also one unit doesn't fit/work on all vehicles. I didn't know such a nice unit was available at a reasonable price. Hopefully none of us need to bleed out brakes often enough to have need for a power bleeder. However, with anti-stop brakes a power bleeder is often a requirement.

I had the same thoughts. If the XJ has anti-stop brakes the complication level of troubleshooting goes way up. Best to cover all the standard hydraulic things first anyway. I would like to add some clarification to your description of the sensor you covered. It is a switch in the proportioning valve that provides a ground to the Brake warning light in the dash. You can simply disconnect the connector on the proportioning valve switch to see if this is the source of the brake light being on. Even if you have anti-stop brakes. When the light is on and you disconnect the connector and the light goes out you do indeed have a hydraulic leak somewhere. Bear in mind too that the parking brake has a switch that also provides a ground to the same light. So if the light stays on after you disconnect the proportioning valve, try moving the parking brake lever to see if the light turns off. One way to tell if your brake warning light is caused by the anti-stop module is to apply the parking brake while watching the light. It will get slightly brighter if the anti-stop module was providing the ground for the light first.
The proportioning valve is an important device in your brake system. Due to the fact that an XJ has drum brakes in the rear and disc brakes in the front. While braking, the weight bias moves forward so that the front brakes handle about 80% of the stopping energy (90% for a motorcycle). Also drum breaks require a residual check valve to keep about 10 psi of pressure on the line after a brake application. This is to help over come return spring pressure in a drum brake. Disc brakes use seal lip elasticity to release brake pressure after you remove your foot off the pedal. The proportioning valve has about a 200-500 psi delay for applying rear brakes so that the rear wheel don't lock up under hard braking. In a hard braking application the brake line hydraulic pressure can easily exceed 5000 psi. And so that an exact balance of maximum stopping power can be obtained from all wheels at gross vehicle weight (GVW). On a dual brakes system as all vehicles have had since the late 60's, a proportioning valve is required. As Mike said if either the front or rear system cannot hold normal hydraulic pressure the shuttle valve in the proportioning valve will move toward the low pressure side and lift the pin on the brake warning light switch providing a ground. The shuttle valve will also shut off any flow to the failed side to somewhat stem the loss of fluid. Some proportioning valves will shut off one front wheel and allow the other three wheels to have full breaking power in the event of a line failure. On some vehicles (not Cherokees) you have to manually push the shuttle valve back into the home position after repairing the leak. Some valves may require bleeding to recenter the shuttle valve. Not on the Cherokee either.
If you have a light on but are sure your brakes are working normally with no leaks, you don't have an anti-stop system. You disconnected the brake warning light connector on the proportioning valve and the light is still on.... Look for a short to ground on the parking brake switch and or the wire on the parking brake and proportioning valve to ground somewhere.