This is frequently related to issues within the control piston mechanism, often caused by contamination scratching the control piston, a weak or damaged control amplifier spring, or other damaged components. Resolution typically requires disassembling the pump to inspect, repair, or replace affected parts, and possibly increasing damping or spring stiffness.

Potential leak sources include:

• Shaft Seal: Damaged or worn-out shaft seal
• Pipe Connections: Loose hose connections, damaged fittings, or aged/cracked sealing washers
• Case Ports: Leaks from variable mechanism cover bolts due to loose bolts or damaged seals
• Case Imperfections: Rarely, sand holes in cast iron casings or poor welds
• High Case Pressure: Excessive internal leakage can pressurize the case, damaging seals

Recommended for: using special HF fluids, operating at viscosity/temperature limits, or for pumps installed vertically (shaft up). Refer to technical documentation for specific flow rates.

• Drain all hydraulic fluid from the pump casing
• Perform thorough cleaning and drying
• Apply rust preventive agent
• Store in a dry, ventilated place