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a proportioning valve that adjusts the rear bias according to how far the suspension is loaded down.

By rear bias, do you mean the amount of brake pressure applied to the rear vs. front brakes? This sounds like what the mechanic was describing, I just thought it sounded like a primitive, mechanical system to prevent rear wheel lockup. Would this respond to the transfer of mass to the front of the vehicle while braking, or does it just deal with cargo load? Is this what the wires out of the differential are for, or would they be for a true ABS?

It sounds like you plan to do the P/S seal yourself. How long will it take?

1992 Mighty Max 4x4
3.0 5-speed
Herculined bed