clark michigan 85 III wont go forward

I was about to buy this loader and have test drove it several times. I was going to buy and drive home (only 3 miles away) so I was playing with brakes trying to see if I could get them to work when it stopped going forward, It drives good in reverse. The owner said this happened couple years ago, and took to a shop and they pulled in and out of the shop a few times just looking at it and it started working. They said it might be the valve sticking. It doesn't get used, he just starts it to keep it running. I drained the trany fluid and checked the screen, It looked good and clean but low. Don't have shop manual for it just a parts manual. Any info on what it might be and were I can find a manual would be appreciated. Thank you

clark michigan 85 III wont go forward