I have a 2007 MTD Gold Hydro lawn tractor. I was cutting the lawn and stopped the tractor and PTO while still seated for 3 minutes then went to move forward and reverse and it wouldn't move. It wanted to move but it seemed like something was golding it back. Then I shut off the tractor and released the hydrostatic transmission valve on the back plate to push it forward and it wouldn't move. What are the potential failure modes? It seems to me that if the hydrostatic transmission valve has been released that something is wrong with the brake?