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Repair & Troubleshooting
Forklift Mast Drifts Down Under Load
Forks that slowly settle under load point to internal leakage — worn lift-cylinder seals, a leaking control valve, or a failing lowering/velocity fuse. Tell internal drift from an external leak first.
— Reviewed by the ForkliftIQ technical team
A mast that slowly lowers on its own while holding a load is losing oil from under the piston back to tank — internal leakage. The usual culprits are worn lift-cylinder seals letting oil bypass the piston, a control (spool) valve that no longer seals in the lower position, or a lowering-control/velocity fuse that isn't holding. The first job is to tell internal drift (no visible oil) from an external leak (oil on the mast or floor), because the fixes are completely different.
Worn lift-cylinder seals — The piston seal inside the lift cylinder lets pressurised oil bypass back to tank, so the load settles even with the valve closed and no oil on the outside.
Leaking control/spool valve — A worn or dirt-scored lift spool doesn't seal fully in the neutral/lower position, bleeding oil off the cylinder and letting the forks creep down.
Failing lowering-control or check valve — The velocity fuse or load-check valve that should hold the mast up passes oil when worn or contaminated, allowing slow drift.
How to diagnose it
1
Confirm it is internal drift: lift a rated load, mark the mast height, and look for any external oil — a clean mast that still settles means internal leakage.
2
Measure the drift rate against the manufacturer's allowance; a small settle over minutes may be within spec, a steady creep is not.
3
Test with the pump off and the valve in neutral — if it still drifts, the leak is past the cylinder seals or check valve, not the pump.
4
Inspect the control valve and its spool for wear or trapped debris; a dirty spool often causes drift on one function only.
5
If the valve holds but the cylinder doesn't, reseal or replace the lift cylinder; renew the lowering-control/velocity fuse if it passes oil.
6
Change the hydraulic filter and confirm clean oil — grit is what scores spools and seals in the first place.
⚠ Safety: Never work, inspect or reach under a mast that drifts while it is holding a load — chock the carriage or fully lower and block the mast first. A cylinder or valve that bleeds down can drop the load without warning.
The mast is losing oil internally — usually worn lift-cylinder seals, a leaking control-valve spool, or a failing lowering/velocity fuse. Oil bypasses back to tank, so the load settles even with the valve closed.
How much mast drift is acceptable?
A very small settle over several minutes can be within the maker's spec, but a steady, visible creep is not. Check the drift rate against the service manual before deciding to repair.
How do I know if it's an internal or external leak?
Lift a load and look for oil on the mast, cylinders or floor. Visible oil is an external leak (fittings, hoses, rod seals); a clean mast that still drifts is internal leakage past the piston seal or valve.
Can dirty hydraulic oil cause mast drift?
Yes — grit scores the valve spool and cylinder seals, and debris can hold a check valve open. Contaminated oil is a common root cause, so change the filter and oil as part of the repair.
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