CReativepart, I have seen many posts on this topic. Just do a search. Many replace the valve with a metal one from Lowes or Home Depot. We are lucky, some had theirs installed backwards. I have only used mine twice and no problems. Some say they can get clogged too.
I wonder if using very warm water would make a difference and clean it out if clogged?
2004 Winnebago Minnie 22e(went to a nice family Oct 3, 2015)
2016 Forester 2701DS (Aug 31, 2015)
2015 Camping 3 trips 6 nights
2016 Camping 9 trips 28 nights