A failure detection apparatus useful with a hydraulic system for maintaining fluid pressure signals present in the system to detect predetermined disparity therebetween. The apparatus includes a movable sleeve mounted within a housing and defining a central bore as well as transverse fluid flow passageways therethrough. A piston is mounted within the bore in the sleeve and normally blocks the fluid flow passageways defined within the sleeve. Fluid pressure signals to be monitored for detection of disparity therbetween are applied to opposite ends of the movable sleeve. Upon movement of the sleeve, fluid under pressure is applied through the passageways and through bores provided in the pistons thereby to cause movement of the piston to apply the fluid under pressure to a shut-off valve and a failure indicator mechanism. The piston latches in a limit position to retain the detection device and the shut-off valve in the actuated position. The operational portions of the system are isolated from the fluid pressures in the failure detection portions thereof.