Parking brake on Toro riding mower seems stuck in on position. Mower starts but will not move.?

It's an automatic. Traction belt is very loose and won't tighten when brake is released (attempted). Release lever for pushing does nothing. Parking brake lever seems to function and lever releases when brake is pushed, but doesn't really release. Should traction belt be tight at all times or is it supposed to loosen when brake is on? This was the first attempt to use mower after winter. I heard there was a brake release lever somewhere at the rear of the machine but can't find it. The model is 71227, serial 220010001.