I do not think there is a problem with your mast. If you are trying to hold a load with no drift for long periods of time, you need something other than a forklift.
every system on any lifttruck has an allowable drift allowance over a period of time
Here is the drift test for this truck from hyster
Mast Vertical Creep
1. Place a capacity load on forks and raise and lower load several times until hydraulic oil has reached 65°C (150°F).
2. Raise forks approximately 1 m (3.3 ft) above ground, put lift controls in neutral position and measure vertical creep during 10 minutes.
3. Mast Vertical Creep must be limited to 100 mm (3.94 in.) in 10 minutes for trucks up to 10 tons lifting capacity.
4. Mast Vertical Creep must be limited to 200 mm (7.87 in.) in 10 minutes for trucks above 10 tons lifting capacity.
5. If mast vertical creep exceeds maximum allowed value, determine if an internal leak is present at lift cylinder or at control valve.
6. Check lift cylinders for internal leaks.
7. Remove load from forks. Install a gate valve in each of separate supply lines between lift cylinder and control valve. Place again a capacity load on forks, raise forks approximately 1 m (3.3 ft) above ground and close both gate valves. If creep speed is similar to the measurement in Step 2, lift cylinders need to be resealed.
8. Check functionality of Emergency Lowering Valve. Replace Emergency Lowering Valve to verify if its replacement solves the problem. If creep speed is still too high, replace relevant control valve section. Individual control valve section components are not separately available.