Originally Posted by old welder
Hey All, Just installed a Cherokee proportioning valve on my TJ with the new 8.8 (rear disc). The valves are identical in all fitting sizes, the extra port on the rear of the TJ valve is plugged on the Cherokee valve, remove the plug and exchange it for the fitting on the TJ valve, everything is the same, took out the plug on the other end, the only difference is the spring length. Suggest not buying anything, remove nut on back side (with plastic cap) and shorten spring by 1/4 of an inch. This allows pressure to get to the rear sooner. (P.S. This valve came off A 1996 Disc/Disc, Cherokee)
The pressure to the rear in either discs or drums is delivered at exactly the same rate up until you hit the brakes in a panic stop. What the spring does is hold pressure on a valve that remains static until the pressure builds enough for the cup to move which then covers a port LIMITING pressure to the rear brakes to prevent or attempt to prevent lock-up in a panic stop.
All you've done by weakening or shortening the spring is let it do that sooner which is probably pretty appropriate for the faster higher weight transfer rate of the shorter wheelbase TJ.
Just know that if you take it too far, you are in fact cutting the total pressure the rear brakes will see and could wind up with them not working at all if you get it low enough.