Why Your Excavator’s Boom Drops Overnight: Hydraulic Leak-Down Explained

When your excavator’s boom sinks slowly while the machine is turned off, the problem is usually internal. This isn’t just gravity at work — it’s hydraulic leak-down, and it means pressure is escaping somewhere it shouldn’t.

What Causes Hydraulic Leak-Down?

  • Worn piston seals: Internal cylinder seals that no longer hold pressure allow fluid to bypass and drop the boom.
  • Leaky control valves: If spool valves or check valves leak internally, they let pressure bleed out of the circuit.
  • Scored or pitted cylinder rods: Damage to rod surfaces leads to accelerated seal wear and internal bypass.

Why It Matters

Aside from being a nuisance, uncontrolled boom movement can cause safety hazards on job sites or during transport. It also points to wear that could eventually result in total hydraulic failure.

How to Diagnose It

  • Conduct a pressure drop test on the cylinder and valve section.
  • Check for external leaks, but remember: internal bypassing often leaves no visible sign.
  • Isolate the cylinder by blocking hydraulic lines and observing whether the boom still drops.

Repair Options

  • Seal kit installation: Most boom cylinder issues are fixed by resealing — especially if no rod damage is present.
  • Cylinder honing or replacement: Severely worn internal surfaces may require more than seals.
  • Valve body rebuild or replacement: If pressure loss comes from the control block, the valve section may need servicing.

Conclusion

If your excavator boom drops overnight, don’t ignore it. Leak-down is a clear sign that your hydraulic system isn’t holding pressure like it should. Professional diagnosis and reseal or valve service are usually all that’s needed to restore full function and safety.

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