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Mistsubishi FG model LP forlift truck forks and carriage slowly drifts down with a load on it. We checked the hydraulic fluid level and it is fine. It only drift down with a heavy pallet on the forks and not touching the controls. You can actually hear it creak when it it going down. It is very slow, but still not safe to use. What is a common cause and repair of this problem on our Mitsubishi ?

Hello and welcome to fixaforklift!!

There are any causes that can cause this, but the most common one that I have found is, inside the lift cylinders there are internal seal's of the cylinder rod, if fluid leaks internally these seals need to be replaced or "re-packing" not too hard to do but does require a technician to perform,

I have also seen check valves inside hydraulic valve wear out to a point that it can cause this, pressure test would be needed to determine this.

Some models also include an emergency down solenoid with a turn valve on top if the valve is open this would also cause to drift down