A lifting and lowering system for a trailer includes one or more extendible and retractable legs, with the legs being provided with springs or the like urging the legs toward their retracted position. The system is a pressurized air/hydraulic system, and includes an air-operated booster valve for extending the legs when loaded. A master valve assembly is provided for controlling supply of air to the system and for allowing retraction of the legs. The master valve and air valve include a common actuator shaft so that the system is selectively connected to pressurized air or vented, as appropriate, when leg extension or retraction occurs. The master valve includes a cam actuator/cam follower piston operated system for selectively providing fluid pressure to a lock valve associated with each leg. The lock valve is biased toward a normally closed position, and is selectively movable to an open position for allowing extension or retraction of the legs.