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Originally Posted by RichardJ

>>So what is it supposed to do on a drum/drum brake system, <<

I'm guessing again, but I imagine the guy was having a problem with the rear brakes locking up. Assuming all the drums were working as they should, I think you are looking at a very aggressive driver with poor brake habits that frequently drive in adverse conditions.

Any truck with no load will have a lot of weight transfer, rear-to-front when braking. That truck has a heavy Big Block and big ass brake booster. You add that to someone that drives too fast in the rain, tailgates and jumps on the brakes at the last minute and that truck might be prone to swapping ends.

Below is an adjustable proportioning valve used on some trucks before ABS brakes became common place. When the bed rises, the arm connected to the axle, adjusts the proportion valve to reduce the pressure to the rear brakes. With a load in the bed, the valve doesn't reduce rear pressure. There is less rear-to-front transfer, the tires are loaded and the wheels don't lock up.

Yes, a Combination Valve would add a warning light switch and also add a Metering Valve for the front brakes.
When you add disc brakes, the truck may have a tendency to nose dive at very low speeds with light brake pressure then a Metering Valve would be very useful. In other words, if you plant your girlfriends face in the dash at red lights, either add a free standing Metering Valve or replace the adjustable prop valve with a Combination valve that has a fixed proportion valve and you should be good and ready for the disc/ drum set up.

The adjustable valve is better than a fixed valve, but probably not needed.

Yeah, it's hard telling what he was up to.

To the OP: When you do the disk swap I would just put the correct combo valve on it and set it up correctly, like Richard said....

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