autodrive

I have a Troy-Bilt bronco 42" with the autodrive system. When I first boought it 6 years ago, the engine would help hold be back from flying down the hill, as long as I had my foot on the gas pedal. Now it won't hold any more and i have to brake and slide down the hill. The "Troy-Bilt authorized servive provider" I took it to last year did nothing to fix my problem. What needs to be fixed or adjusted to make it safe for me to mow my front yard again.