Not sure if a 853 has similar hydraulics to a 743, but theres a good video on utube by DannysCam, how to fix Bobcat that doesnt move. Theres an internal filter in one of valve bodys under cab, that clogs up with junk and limits drive. Might be worth a look if you've run out of ideas. The only down side to that is if its clogged with filings, you've got bigger problems.