I have been using my 1952 Ford 8N to run a 4' Bush-Hog Squealer lately. It has been extremely hot here and I find that after I have been running for some time that, when I attempt push the handle down to lower it, that there is considerable resistance. Everything still works as it should but there is resistance there which is not there when starting out. Hydraulics were rebuilt about five years ago with fresh fluid and also I changed the fluid last year. Using proper 90 weight oil as was recommended here. Dip stick for fluid shows it is up to correct level and does not show any water mixed with it. Tractor sits covered when not being used. Your thoughts?