If the check chains are going tight when you lift, they shouldn't do that. This can be caused by chains too short or that the attachment brackets around the PTO are fitted upside down so that the Chain attachment point is below the centre line of the PTO shaft, if this is the case they should be re-attached with the chains fitted above the centre line of the PTO. Washing out with Diesel is not a good idea as it will not all drain out and this will thin your new oil. When the 3 point is sticking at the top this is because the chains are too short and this is causing the Control Valve to stick, because it is trying to lift and being prevented by the Check Chains and it is building up a pressure that is causing the Control Valve to stick. You need to free the Control Valve by removing the Plate with the dipstick on the right below the seat. You will see the Control Fork hanging down from the top and going into the oil. You should use a screwdriver and move the Control Fork, back-wards and for-wards a few time GENTLY. It should move a couple of inches. DON'T PUT YOUR HANDS IN THE OIL< One the Control Valve is free you need to allow the oil to warm up so that any old oil will be cleared from the Valve and then it will work easily. Don't user the hydraulic s while the oil is cold to start with or it may stick again.
It will possibly also need the Control Lever re-setting
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