Primary reasons are:

• Insufficient Inflow: Caused by high suction line resistance, low oil level, excessive pump speed, or a clogged inlet strainer/suction line.
• Excessive Leakage: Worn internal parts (pistons, cylinder block, port plate) leading to high internal leakage.
• Swash Plate Angle: The swash plate angle might be set too small, minimizing displacement.

This is often due to the control mechanism being off its neutral point. The cause can be a misadjusted, loose, or damaged controller. The solution is to re-zero, tighten, or replace the controller.

Flow instability, often accompanied by pressure fluctuations, is frequently related to issues within the control piston mechanism. This can be due to contamination scratching the control piston, a weak or damaged control amplifier spring, or other damaged components. Addressing this usually requires disassembling the pump to inspect, repair, or replace the affected parts, and possibly increasing damping or spring stiffness.

External leaks can originate from:

• Shaft Seal: Damaged or worn-out shaft seal.
• Pipe Connections: Loose hose connections, damaged fittings, or aged/cracked sealing washers.