Abstract: A hydrostatic system for controlling ventilation curtains in animal enclosures. A fluid-powered locking system is coupled by hoses to the winch assembly which controls elevation of the ventilation curtains. The locking system comprises a spring-biased piston associated with a rigid cylindrical casing. The casing is sealed by a rigid head and houses a flexible diaphragm which selectively contacts the piston. Fluid pressure is supplied to the casing via hoses coupled to the water line or other fluid source. Fluid entering the casing compresses the diaphragm, which in turn compresses the piston. The rigid piston rod is urged outwardly through the cylinder into frictional engagement with the winch spool. The cylinder at least temporarily locks the spool and thus the curtains at the desired position. A direct current powered solenoid valve maintains pressure to the system. When A.C.