Abstract: A hydropneumatic system, especially for operating landing gear of truck trailers, is operable from a source of pressurized air. The system includes separate first and second hydraulic fluid sources. Dual hydraulic cylinders are operated by the fluid. Each has a piston and an extensible member connected to the piston for providing extension and retraction of the gear legs in response to hydraulic pressure in chambers on opposite sides of the pistons. An air valve provides the pressurized air to one of the fluid sources for enabling it to provide hydraulic fluid. An air-operated hydraulic pressure boosting pump is selectively operable to boost the hydraulic fluid pressure thus provided to much greater than air pressure.