How about the hydrostatic steering?
Is it smooth?

How about the other functions (tilt, siedeshift).
Are they working properly?

Are you able to measure the pressure in the working hydraulics?
You should have (depending upon the truck capacity and the type of the mast) something between 165 and 250 bar.

Is the pressure limiting valve properly set?
The valve is located low, on the first, control section of the control valve. You may check, if it didn't stuck.

Are there no nozzles in the control valve lift/lower section?
Remove the hydraulic hoses from the first section and look inside.
If you find some rings with small diameter hole in it - remove them.

Well.. enough for now.

Good luck