My legend seems to be getting stuck at 2 certain points at my steps. It has never done this before. I replaced one set of tread tires as the ones on the back had no more tread. I switched the older ones to the back and put new ones on front. It climbs nicely and I have seen it go up steps and float away. However there seems to be two specific area at my steps that it will just sit. Any suggestions? How can I test the back up valve? Thanks!