a lift axle suspension for a tandem trailer has first and second suspension air bags for first and second axles . there is a lift air bag that , when pressurized , applies a lifting force to lift the second axle relative to the first axle . a first pneumatic circuit supplies compressed air to the first suspension air bag and a diverter valve and is controlled by a height selector valve that is opened and closed based on the ride height of the trailer . the diverter valve selectively supplies compressed air to the second suspension air bag and a pressure regulator that limits air pressure to a predetermined pressure . a second pneumatic supplies compressed air to the lift air bag and is controlled by a pilot valve .