What is the purpose of a 'case drain' line on a hydraulic pump?

The purpose of a 'case drain' line on a hydraulic pump is to provide a low-pressure return path for any small amount of internal leakage that occurs within the pump housing (case). Hydraulic pumps, even when new, have some degree of internal leakage. This leakage is due to the necessary clearances between the moving parts, such as pistons, gears, and bearings. The leakage fluid, if allowed to accumulate within the pump housing, would cause the pressure inside the housing to increase. This increased pressure can damage the pump's seals, leading to external leaks and potential pump failure. The case drain line provides a dedicated path for this leakage fluid to return to the hydraulic reservoir (tank) at low pressure. This prevents pressure buildup within the pump housing and protects the pump's seals. The case drain line is typically connected to the pump housing and routed directly to the reservoir, often through a dedicated port. It is important that the case drain line is not connected to a high-pressure return line, as this would defeat its purpose. Monitoring the flow rate of the case drain line can be a valuable diagnostic tool. A gradual increase in case drain flow over time indicates increasing internal leakage within the pump, which can be a sign of wear or damage. A sudden increase in case drain flow can indicate a more serious problem, such as a broken seal or a damaged component. Therefore, the case drain line serves as both a protective mechanism and a diagnostic indicator for the hydraulic pump.