Abstract: Three embodiments of a hydraulic safety mechanism are adapted to adjust the speed setting for an engine upon a reduction of pressure in a chamber, due for example, to loss of lubricating pressure, overspeeding of the engine or overheating, for example. In each embodiment, a speed control element for the engine is maintained in a normal operating position by a piston and cylinder combination which hydraulically blocks the chamber from the fluid drain in a normal operating position, the piston responding to reduced pressure in the chamber by communicating the chamber with the drain the piston thereby being hydraulically unlatched and permitting the speed control element to be repositioned to a reduced speed setting. A restrictive orifice, preferably of variable size and self-cleaning construction, regulates communication between the hydraulic circuit and the chamber. In another embodiment the safety mechanism includes a solenoid and accumulator chamber to adapt it for remote operation.