The piston of a pneumatic actuator is releasably locked in extended position by a hydraulic device responsive to the air pressure in the actuator cylinder. A hydraulic supply reservoir feeds hydraulic fluid through a check valve to a chamber containing a displacement piston attached to the actuator piston so that the pistons are permitted to move to extended position but prevented from returning. A controllable release valve bypassing the check valve is provided to allow return of the hydraulic fluid from the chamber to the reservoir to permit retraction of the actuator piston.