LSPV = Load Sensing Proportioning Valve, correct?
If I don't use my truck as a truck (no load) why do I need this thing?
Definitely going bye-bye on my 1983 (I'm going rear disc).

 

The load refered to is on the rear axle. There is a change in the amount of load on the rear axle during ordinary braking as the weight load of the vehicle shifts forward toward the front axle, due to the pitching forward of the vehicle(higher in the rear, lower in the front). The LSPV can aid in preventing rear wheel lockup from this effect by proportioning less braking pressure to the rear brakes. Rather loaded with cargo or not, drum brake or not. Though the valve actually "works" less when the rear of the vehicle is fully loaded as the additional weight is all that's required for keeping the rear wheels from locking up when braking hard. This is because with more weight in the rear, less weight is thrown forward(vehicle pitches forward less when braking), leaving more load on the rear axle/rear brakes. When that load is sensed by the valve, it limits the valve from proportioning less braking force to the rear brakes, as that's no longer necessary to prevent lockup. Meaning you "need" it less when at max carrying capacity, and more when unloaded.