You had mentioned you bought your walker in dec 2010. You should look a the month it was manufactured (newer machines say month and year it was built by serial #).
All hydrostatic built after sept 2010 were supposed to have added check valve to lessen the chances of spiking hydro according to walker/eaton. Same design prior 2006 brought back by eaton. However sales being down the walker dealers/distributors sell 6 month old machines on average. Do you run a speed up pulley on it?