Komatsu Bulldozers D31 D39 D51 Hydraulic Control Valve Assemblies Common Faults & Troubleshooting Specific action is weak / not working: Stuck spool in that section, spool/sleeve wear causing internal leakage, damaged O-rings. First measure the outlet pressure of that section to diagnose. All actions are weak: Usually caused by insufficient main pump pressure or a failed main Relief Valve pressure setting. Check system pressure first before disassembling the valve. Erratic /