The lock valve combines a Kepsel® Cartridge Insert Check with a rugged pilot operator. They free flow from valve port to cylinder port (or to portion of circuit) and “lock” reverse flow until pilot pressure is applied to unlock the internal check valve and allow reverse flow. Kepner's Flexible Seal Seat™ provides positive bubble-tight locking to prevent load drift or circuit creep, and answers the need where internal leakage could adversely affect performance such as with clamping cylinders or outrigger stabilizer jacks. Standard and specialty valve designs handle a wide variety of applications for dependable position and load holding, including safety holding against line rupture or control valve leakage.