The rear brakes on this Jeep have never worked. When on icy roads, the rear brakes won't even hold the vehicle still at stop lights, I have to put the trans in neutral. Dealers have told me this is normal, but it can't be. I've thought about adding an adjustable proportioning valve. It's really dangerous on wet roads because the front brakes lock up quickly. The vehicle is completely stock other than 31" tires rather than the stock 30" tires. Even with the stock tires it did the same thing. Is the adjustable proportioning valve the way to go?