In June 2017, I deleted the rear load sensing valve from my 1991 Comanche, and have documented the process here.
The rear load sensing valve system is comprised of (1) an MJ-specific distribution block at the master cylinder which provides full pressure to the rear brakes; (2) an inline proportioning valve for the rear brakes (load sensing valve) located above the rear axle which is actuated by (3) a bar connected to the rear axle; (4) two hard lines running from the distribution block to the load sensing valve; (5) one hard line running from the load sensing valve to the rear brake drop line.
The purpose of the rear load sensing valve is to adjust braking pressure applied to the rear brakes under different conditions. The rear load sensing valve is attached to the vehicle frame above the rear axle, to the driver's side. A bar connects a lever on the valve with the rear axle using a bracket on the differential cover. When the rear axle moves away from the frame (sudden stop, where the front end of the vehicle dips), as the height of the rear of the truck increases, the bar pulls the lever on the valve in a downward direction, resulting in the load sensing valve reducing pressure for the rear brakes. When the rear axle moves closer to the frame (load in the truck bed), as the height of the rear of the truck decreases, the bar pushes the lever on the valve in an upward direction, resulting in the load sensing valve increasing pressure for the rear brakes.
Other Recipes, Why a YJ Prop Valve?
Some recipes for load sensing valve deletion call for retaining the MJ-specific distribution block and plugging the bottom-front emergency bypass port. This method results in 100% braking pressure being pushed to the rear brakes at all times, and can cause premature lockup of the rear wheels, if too much pressure is applied by the driver to the brake pedal. Retaining the distribution block is generally acceptable for the older drivers who grew up without proportioned braking systems, and know how to apply brakes without proportioning.
This recipe replaces the distribution block with a proportioning valve from a 1987-1995 Jeep Wrangler (YJ). The YJ proportioning valve limits braking pressure sent to the rear by 20%, thereby sending 80% pressure to the rear brakes. This proportioning valve is widely accepted to be nearly identical in function to those found in XJ and ZJ vehicles. It has been proven to work efficiently/correctly in vehicles which have rear drum brakes, as well those that have been converted to rear disc brakes.
The rear load sensing valve is part of the vehicle's braking system. Modify at your own risk, this documentation is provided for informational purposes only.
In my case, I decided to delete the valve because it wasn't operational. The bracket the valve mounts to was severely bent, the bar connecting it to the axle was missing, and I had no idea if it actually worked or not. Here's a pic of what I started with:
And the MJ-specific distribution block, in stock format is pictured below. Note that the line entering the front port on the distribution block is the main line running to the load sensing valve, and the line entering the bottom-front port on the distribution block is an emergency bypass line which is not proportioned. That line sends full braking pressure to the rear in the event the front braking system fails.
Here's a pic of our donor proportioning valve, from a 1995 Wrangler, still attached to the master cylinder it was pulled with.
And here's a pic of the MJ-specific distribution block on the left, and the YJ proportioning valve on the right. Don't get them confused, they're very similar looking. The YJ valve has the front port plugged.
The first step in replacing the load sensing valve is to bleed all brake fluid from your braking system. There should be no fluid in the system prior to disconnecting anything.
Next, remove the MJ-specific distribution block from under the master cylinder. Due to flex in the hard lines attached to it, and how tight the connections can be, I recommend using tools which will provide you the leverage you need, like large vice-grips and a proper wrench. Remove all six of the hard line connections running to the distribution block and remove it.
Next, cut the hard brake line that was plugged into the front port of the distribution block (the main hard line) into pieces to remove it. The entire line will be discarded. Start at the front of the truck and work your way towards the rear, cutting a foot or two of the line at a time. You'll want to cut the line in this fashion as routing under the truck and past the gas tank is impossible to follow. You are cutting that line completely out, all the way back to where it plugs into the rear load sensing valve.
Once the main hard line is removed, take a look at your rear load sensing valve. There will be two uncut hard lines still plugged into it. One line runs from the front of the vehicle to the load sensing valve (the emergency bypass line), and one that runs back to the rear brake drop line. Disconnect both from the rear load sensing valve using a wrench and carefully bend out of the way slightly.
Locate the attachment point for the load sensing valve's rod on the rear axle differential cover and detach it. At this point the rear load sensing valve can come out. Unbolt the bracket it's mounted to from the truck frame. The bolts will likely break rather than come out freely. Discard the entire load sensing valve assembly, or send it to a club member who wants a backup. I got a nice set of lug nuts in trade for mine.
Now you have an option. You can either use a coupler to attach the two remaining hard lines where the load sensing valve was, or you can remove both lines and run a single hard line from the front of the truck to the rear brake drop line. My hard lines were in good shape, so I opted to couple them...
Next, use a 3/16" brake line coupler, available at any auto parts store (NAPA part number G60693-0303), to connect the two hard lines that you disconnected from the load sensing valve. You are connecting the remaining line that runs from the front all the way to the rear brake drop line. Here is a pic from the front side of the frame crossmember showing the two lines connected with a coupler:
And here is a pic from the rear side of the frame crossmember showing the connection to the rear brake drop line:
Once you have the rear sorted, return to the front of the vehicle and install the YJ proportioning valve. All of port locations on the YJ proportioning valve match the MJ-specific distribution block's port locations, so connect the hard lines back up the way you disconnected them. You'll be missing the one hard line you removed earlier, obviously. Here's a pic of the YJ proportioning valve installed, with all hard lines connected:
When finished, fill and bleed your entire braking system (front and rear), then perform several tests, starting at extremely slow speeds to test proper functioning.
I deleted the rear load sensing valve from my Comanche eight months ago as of this writing. I am happy to report that braking using this recipe is as you would expect, without any problems, issues or caveats. I have only experienced rear brake lockup when using full braking pressure, going downhill on wet surfaces with radial tires. Under the same conditions, the truck stops without locking up the rear brakes when using all terrain tires.