This mower has a problem with the rear wheels locking up. They go fine forward, but when I try to pull the mower in reverse the rear wheels are locked and won't turn freely. I have taken it to my dealer previously and they weren't very helpful, charged me $30 to "oil" them and they worked fine for about 4 days. Now I'm back to the same issue. Any thing I can do to fix this very annoying issue? Thanks for any help!