The main drain seems to be plugged on my in-ground pool built in the '70s with a sand filter and a push-pull reversing valve. When I shut off the skimmer line the pressure drops to 0 and the pump sound changes (like cavitation). I've tried a Drain King on a garden hose from the pump-house side with no luck. I've tried vacuuming the main drain with the skimmer-connected vacuum and the main line shut off and no luck. It seems like there is a check-value (backflow preventer) in the line but it is not accessible. (Can anyone confirm that pools were made that way--inaccessible underground check-valve?)

What are my options? Any ideas? Does a Roto-Rooter-like snake from the bottom of the pool work (until it gets to the check-valve)?