Hy handymanmac, basically an all drum system doesn't need a proportioning valve, the reason you're getting rear brake lock up is because of weight transfer, what little weight there is in the rear of a pickup. I would rather see you use a weight adjusting proportioning valve, the type that the manfacturers use to vary the amount of rear braking effort based on the weight at the back of the truck. Once you "dial in" an adjustable proportioning valve, that is the maximum braking effort you will get in the rear reguardless of load, hope that helps,
