Do you have a single or double channel? With my double channel setup if something made the truck lean it would let air out of one side and put air in the other to keep the pressure constant. Anything that changes the weight on the rear end resulted in a pressure adjustment, quite often the opposite of what I really wanted. Putting a load on the back compresses the air bags, raising the pressure, and the system releases air to maintain the set pressure when I really need air added to get back to the desired height. I am not saying that this is the problem. It does sound like you have a leak but these are things to consider as well.