The Proportioning valve is designed to keep the rear brakes at a lower pressure than the front, and also to provide pressure to either the front or rear, if one system fails. If the pressure in the front drops too low, the rear brakes will take the full force of the braking action (and lock up). Some are also designed to reduce the amount of air let back into the system if a leak should occur. Cherokees are a very heavy vehicle, and the braking system on the older ones were hard to lock up. With the recent problem of having the rear brakes lock up on you, I would suggest that you very carefully check all of your lines for pinhole leaks. You can also upgrade your rotors to drilled and/or slotted, to help to improve the performance. Also check for proper adjustment of the rear brakes and make sure all caliper/shoe contact points are smooth, well polished, and lubricated with a grease designed for brake systems. Use 120 grit sand paper to break any glaze on rotors, or have them turned if they show wear.
Thursday, December 9th, 2010 AT 11:32 PM