pto issues

New owner of a used MF1533L, year unknown, 599 hours. Engine starts easy and runs smooth but it has no power in the rear-end but Front End Loader works fine. With nothing attached on the back, RPM set to 540-mark, turning PTO on (and the lever either already in 540 or starting in Neutral then...

New to the forum and I have googled, brought in a certified mechanic who replaced the PTO selenoid and also the tractor now has fresh hydraulic oil and filter. Screen was extremely clean. So I come here to ask you all. But my issue is when I'm disc mowing, the mower will get up to speed and work...