Ford 860 3 point lift arms sometimes will not function after sitting for a few days. It does lift when I first start the tractor, lifts to whatever level it was left at. But then often will not lift further, and when I drop the hydraulics it will not lift back up. I can get it to work again by bleeding the front plug on the (round piston) pump. I have changed the fluid twice, it was quite dark the first time, still a little dirty the second time, looks ok now. I did have problems once last year; it stuck up, would not go down. I tapped the top cover with a block of wood and a hammer. That released it, and it worked ok until last month. It does have an extra hydraulic valve on the top cover plate, it powers a hydraulic top link. When the lift arms will not lift, the other valve also does not work. All advice is appreciated.