I just bought and older b2150 4x4 tractor, got it home and toying around in yard I have noticed it seems to struggle to pull its self. I got my neighbors bush hog last night and with the extra weight it will simply stop pulling at any incline. With this said I took tractor and pushed up against a tree and in low range it will not even try to turn rear tires in 2 wd. Kinda sounds like fluid is bypassing in transmission. I changed fluid and filter and found one screen to clean. With this done it has made no difference at all, any words of advice would be greatly appreciated. First Kubota and it's a week old to me!