BillB , I have a standpipe like your's but with only one valve which is so rusted it won't open . I have owned the pond for 6 years and have no idea how old it is and have had to backflush it with a trash pump pulling water out of the pond and into the top of the standpipe it worked just fine to unstop it , maybe will work for you.
Any body out there know someone that could replace the valve without draining the pond--- Thanks for any help
