[Ed t]

I have a 300 tractor that has been sitting for awhile , I have it running and starting to tinker with it. My issue is that when I put any pressure or climb any hill it feels and sounds like a gear slipping but reverse works fine. Oil and filter have been changed and both by pass valves have been cleaned with no change. If someone could please point me in the right direction. I got this tractor from my brother and it worked perfectly when it was parked