I had a similar problem and checked various threads. My machine had 560 hours and is just shy of five years old. I had never replaced the drive belt and figured it would be an easy fix. Put a new one in...the old one was noticeably worn. But no improvement at all. So I checked various threads, did the steps above to bleed, checked the linkage (nothing loose) and still no improvement. I took the right rear wheel off to inspect the rear end closer and voila, there was the problem. The frame was broken right in front of the transaxle. I did a search for that online and found that this was quite a common problem. Luckily it was three weeks before the 5-year warranty was up on the frame.