Abstract: A differential valve assembly that maintains an outlet pressure at a predetermined difference from an inlet pressure. It can be used to keep a large vehicle frame horizontal despite a permanent extra load on a first side of the frame. In that case, the system includes a simple air refill valve device (52) that connects a pressured air supply S to the first air bag (12), to vary the first air bag pressure so as to maintain the frame at a predetermined height above a location on the axle. The system also includes a differential valve assembly (54) that includes a first valve (72) that can fill the second air bag if its pressure is too low, and a second valve (80) that can vent the second air bag to the atmosphere if the second air bag pressure is too high.