A component that holds a constant and steady suction, a pressure lower than that of the atmosphere, on a system. Some gyroscopic instruments used in an aircraft are operated by air pulled into the instrument case by a suction produced by an engine-driven vacuum pump.

A suction relief valve in the line between the pump and the instrument case has a spring-loaded poppet that remains closed until the pressure becomes low enough to allow outside air pressure to force the poppet off its seat, then outside air is taken into the pump through the valve, rather than through the instrument case. This air entering the system holds the pressure inside the instrument case at the value for which the suction relief valve is set.