My 1955 Ford 860 3 point lift arms will not raise. The arms can be moved up and down by hand freely with tractor running or off. Used it 3 weeks ago and worked fine. Temp was much colder at 30 degrees when issue arose. Fluid is full and was changed a year ago. Had a sagging issue last year and replaced the cylinder piston and gaskets, also had the hydraulic cylinder honed. Worked fine for 10 hours before this issue occurred. Ever since I've owned the tractor I noticed that I get no movement from the 3 point arms until the lever is raised 1/2 way but it does raise to the proper height. Ran the tractor with the plug removed from the top of the hydraulic pump and got no fluid discharge. Also poured a little fluid in the plug hole but that made no difference. Any ideas? I'm a first time tractor owner and well versed on tractors. Thanks.