zetor 8011 hydraulics

My problem is the hydraulics in my 8011, my hydraulics would lift perfectly 5 days in a row but on the 6th day they would not lift. Cant find any reason for this problem. Gearbox full of oil pump has been taken out and checked and fine. no sign of any oil leaks. Filters checked. Can anybody help me with this problem.

Thanks

There is a plug on the rear of the 3pt hitch housing on top of the rear axle. If it has one, the free return is usually plugged in there. Take the bung or the return line out and tap with a screwdriver and a hammer on the rod or bolt inside the 3pt hitch to free it.

Its usually a stuck valve.

Check the flow control knob hasn't got turned in. It's the round knob behind the seat on older tractors or under the seat on the bulkhead on later ones.
The other cause can be the pump has got airlocked. you may have to face the tractor down the steepest slope you can find to get the oil to gravity fall to the pump, or else slacken the big banjo bolt on the pump, both with the engine running.