My 721 has a problem when driving slow forward it often start to go backwards at the right hand side. Then the directions are mixed up, moving right lever forward will make right side run backward and if I pull the lever backward it will make the right side go forward !? If I go slow again the direction shift to ok again. As long as I pull the lever firmly forward then no problem occurs. Both sides always has god drive power, even when the direction is messed up at the right side. I think hydraulic motor/pump is quite soundy when driving. I have read something about a bronze filter on some models, is it a filter like that on 721 and if it is, where is it located on this model? Maybe a clogged bronze filter is my problem? Any other suggestions?