I had a 185 hydro like that. The hydro slowly faded away, to where it was unable to move. The little pistons that run on the swash plate inside can wear out, just like piston rings. No compression to drive the mower. I found a used one and it lasted for years. Ive also rebuilt a hydro in a customer's 455. Unfortunately your hydro pump is a sealed unit, and its too old for new parts. Ebay will help for old parts.