Main Drain Clogged on Old Pool

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)?

there is a small snake/ sewer machine made with a camera on it ,rent one or call plumber with one.. you shouls be able to push through a swing check valve , but not a pressure check , personally I dont think they would bury one

does your main drain tie into the bottom of the skimmer?

Thanks for the replies.
The main drain and skimmer are separate until just before they get to the pump. Each has a ball shut-off valve after they enter the pump house, then join.
Do you think a snake could go through the ball vales and 90° elbows?

I have had good success with using Pressure Drain Openers... have used one on both 1 1/2" pvc pipe and 2". They are soft rubber bulbs that hook up to the garden hose, are put into the pipe, and then water is turned on causing them to expand inside the pipe, and then remove clog by trapped pressure. I would think one would easily fit through the open ball valve into the pvc. Worth a try, and very inexpensive... they've never let me down.
http://nationalsupplynetwork.com/ass...ges/465348.jpg

Thanks. I would have thought that would work, also. I did use one of those (this one is called a Drain King). It did it's thing, made the pulsing sound, expanding and releasing. I let the hose run for 20 minutes with no success. That's when I got the idea that maybe there was a check valve in the line somewhere. But it could also just be pollen and fir needles caked up. Lots of fir tree branches over the pool. If pollen sits on the bottom of the pool for more than about 5 days, our robot-vacuum won't even get it off. I have to used the stainless-steel brush on it. So I know that pollen can stick and harden.