Isn't there a tensioning pulley somewhere that applies and releases tension on the belt when engaged or disengaged? If that pulley is backed all the way off and there is still too much tension on the belt, then the belt is too short or the blade brake isn't working correctly.

yes. there is a pulley that can be moved. the belt turns 1 blade pulley. there is a 2nd ribbed belt that connects both blades. yes i think the tension should drop at stop and motor pulley should slip on belt and belt should not be moving at stop. the blade brake does work as that blade pulley is not moving at stop. the belt is moving and rubbing on blade brake bracket. only an issue when not mowing. works fine when moving/mowing.

If you bought "hardware store belts" that was your first mistake! If original equipment belts are available, and you didn't buy THEM-thats the second mistake! The third mistake would be not buying original belts, because they are a matched set. RJ