XJ Cherokee Conversion to WJ Front Brakes & Steering
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By: Michael J. Rollins - 4/2003

XJ Cherokee Conversion to WJ Front Brakes & Steering
Part 2 - Proportioning Valve

Introduction | Part 1 Booster/Master Cylinder | Part 2 Proportioning Valve | Part 3 Knuckles/Steering/Brakes

This is the second article in a series describing the addition of year 2000 WJ Grand Cherokee steering and brake components to my 1988 XJ Cherokee. In this part, I describe the brake proportioning valve, its function, and modification to adjust for my upgraded brake system.

The proportioning valve controls pressure applied to the brakes to prevent premature brake lock up. These valves are vehicle/model/setup specific for maximum braking control. Earlier, I added a new rear axle that has disc brakes, so I did this modification to allow the rear disc brakes to do their job and help overall braking. I recommend either swapping in a proportioning valve that is set up for rear discs or adding a adjustable proportioning valve to prevent premature rear brake lockup.



Disassembling the proportioning valve. Disassembling the proportioning valve. Disassembling the proportioning valve.
Disassembling the proportioning valve.

Proportioning Valve Described

The proportioning valve has two valves in it. One is a shuttle valve that separates the front and rear brake pressure circuits, and activates the pressure warning switch. The second is a pressure metering valve for the rear brakes. The work I am doing is to the pressure metering valve, and modification to this valve will not connect the front and rear brake circuits. If one half of the brake system fails due to a leak, you will still have reduced braking of the other half just as original. The brake pressure warning light will also operate as originally designed in the event of pressure loss in one circuit.

Removing the spring and pressure metering valve. Removing the spring and pressure metering valve. Removing the spring and pressure metering valve.
Removing the spring and pressure metering valve.

The master cylinder is also split into two circuits. The front bore (as sitting in the driver's seat) is for the rear brakes and the rear bore is for the front brakes. All this modification will do is allow the full pressure to the rear brakes from the front bore of the master cylinder. This increased rear brake pressure could lead to premature rear lockup depending on your setup. With the 33" tires I am running it seems to be fine without a adjustable proportioning valve. I do not recommend doing this modification if you still have rear drum brakes. Proceed only if you have switched to rear disc brakes, and only if you understand the ramifications of the change.

Modification of the Assembly

I started by using a large adjustable wrench to hold the body of the prop valve while I unscrew the pressure hold-off valve. Then using another wrench, I unscrewed the nut holding it all in. Be careful as it is under spring pressure. Loosen it with the wrench and do the rest by hand. This spring is what sets the brake bias front and rear. Once the nut has been removed you then pull the spring out. Using small pliers, pull the valve out being careful not to scratch it with the pliers. You can see the little rubber seal at the end. Remove the seal and then reassemble everything else in reverse order. If you are not going to continue on with the next modification in this series, then you now need to bleed the brake system.
The pressure metering valve and rubber seal. The pressure metering valve and rubber seal.
The pressure metering valve and rubber seal.

This part of the project is now done. The final, largest part is the replacement of the stock front knuckles and brakes with custom pieces from the WJ and TJ.

Introduction | Part 1 Booster/Master Cylinder | Part 2 Proportioning Valve | Part 3 Knuckles/Steering/Brakes

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