Hmmm ...
It appears that my understanding of how the front valve in the MJ works was incomplete ... and still is.
In the XJ, it is a "combination" valve, serving as both the brake system warning sensor and as the proportioning valve. The "nose" has a removable plug, behind which is a spring-loaded plunger with an O-ring. It's clear that there is some functionality going on inside there.
On the MJ valve, there is no removable plug in the front. I believed that the rear circuit (the entire valve, in fact) acted simply as a junction block, serving as the brake system warning device but not providing any proportioning function, since on the MJ that's done at the rear by the load-sensing valve.
So far, so good. But I just read the MJ FSM and it appears the front valve does have an additional function -- and I can't figure out how it works because there are no diagrams. The thing isn't supposed to be serviced, only replaced. Note that in the XJ FSM this device is referred to as a "combination" valve, whereas in the MJ FSM it is referred to as a "by-pass differential" valve.
Here's what it says:
MJ FSM said:
VALVE OPERATION
The by-pass pressure differential valve activates the brake warning lamp in the instrument panel if a pressure loss in the front or rear hydraulic system should occur. This feature is provided as a means to alert the driver of a system malfunction.
The by-pass pressure differential valve contains a piston and a plunger-type switch. If a pressure loss occurs in either the front or rear hydraulic brake system, a system differential of 483 to 2068 kPa (70 to 300 psi) will cause the piston to move toward the low pressure side of the valve.
Also, if the valve senses a loss in pressure in the front system, it will direct fluid pressure to the rear wheel cylinders, by-passing the height-sensing proportioning valve.
The last sentence was put in bold by me, for emphasis, because this is new information for me. I have no idea how the valve does this, but it suggests that the second line that I (and other folks) have always thought of as a "return" line is NOT a return line, but a by-pass line that overrides the normal proportioning function.
Since I don't know how to determine how this valve works internally, the best advice I can offer for an MJ with rear drums would be to take it out and use an XJ combo valve. In the past I believed you could just plug the front (nose) outlet and that the MJ valve was open internally, just acting as a distribution block. Now I don't know.
For an MJ with rear discs, it may make sense to use an XJ combo valve and modify it the same as would be done on an XJ with rear discs, or to use a valve from a ZJ that was equipped with rear discs from the factory.
I think I need to get a couple of MJ valves from the u-pull yard, and cut one open to see what makes it tick.