Forklift forks creeping down? Rule out the valve before the cylinder
A mast that won't hold a load is a safety problem, and the lift cylinder usually takes the blame. Sometimes it's the valve. A few checks can tell you which one before anybody orders seals.
Drift is when the forks or the carriage come down slowly on their own with the lever in neutral. A little movement over a long time can be normal, and your operator's manual says how much is acceptable for your truck. If you can see it moving while you watch, that's not normal, and it won't fix itself.
Safety first: don't stand or reach under raised forks while you check this. Lower them, or block the carriage, before anyone puts a hand near the mast.
Three usual suspects
The oil holding the load up can only go three places: out of the cylinder, past the seals inside the cylinder, or back through the control valve to the tank. A leaking hose or fitting counts as the first one, and it's the easiest to find.
- External leak: a wet rod, oil running down the cylinder, a puddle under the mast, a hose or fitting that's weeping.
- Seals inside the cylinder letting oil by.
- The control valve spool, or a holding or lowering valve, not sealing in neutral.
Start with the easy check
Clean the lift cylinders and the hoses so you start dry. Raise the carriage a couple of feet with a normal load, put a piece of tape on the mast as a mark, and shut the truck off. Then watch the mark and look at the cylinder.
If oil shows up on the rod or around the gland while it drifts, you've found it. The rod seal is done, and the next thing to look at is the chrome, because a pitted rod chews through new seals.
If everything stays dry and it still drifts, the oil is going somewhere inside the system. On a lot of forklift masts that points to the valve. Not always: some lift cylinders have a piston seal that can leak internally and send oil back to the tank without a drop showing outside. It's worth confirming before you pay to rebuild a cylinder that might not be the problem.
Tilt drift is a different story
If the mast tilts forward on its own, look at the tilt cylinders. They're double-acting, so worn piston seals can let oil cross from one side to the other without leaking outside. The valve can still be the cause, but with tilt drift the cylinders deserve a close look.
When to call
If you can see the drift, park the truck until it's checked. A load coming down on its own in an aisle is how people get hurt. We work on forklift masts on-site across the Denver metro, and when the cylinder is the problem we reseal or rebuild it in the shop. More on our forklift repair in Denver and our hydraulic cylinder repair.