I have a 1979 Ford 1900 4WD tractor with add on Bush Hog loader. Loader was acting sluggish so I changed hydraulic filter and also change hyd. oil. After this was done loader and 3 pt. hitch would not operate. I bled system and was able to make hydraulics operate. After a few cycles of the hydraulics the system seemed to catch air again. Repeated this process several times and the system seems to keep losing it's prime. I need help.