When the dirt is deposited on the surface of the filter cartridge, the differential pressure between the inlet and outlet of the hydraulic filter element increases. The bypass piston senses a pressure difference. The piston is held by a spring. When the element needs to be cleaned, the differential pressure is high enough to compress the spring and force the piston away from the seat. The movement of the piston points the telltale indicator to the “need to clean” position.