You know my truck has some sort of add a leafs in the rear and its a bit taller than stock, so that should make the valve think there is less weight over the rear tires and it should have less rear braking power, but everytime I've had to hammer on the brakes, the rear tires always lock. It's kinda annoying..
WT, you say yours is in the full upright? Wouldn't that simulate an extreme load in the bed as if the rear suspension was riding on the bump stops, and so more braking power would go to the rear?