Yes, capping the inlet would be the best, but will his county like the idea of it taking a diver to be able to open it up if necessary on short notice?
I'll throw out another idea. How about another valve on the backside. I would tend to do that instead of rigging a way to pump the water back every day. It don't solve the problem of pressure inside the pipe.
Is it possible that the current valve is stuck in a slightly open position and won't seal closed? Possibly some junk/sediment. If you open the valve up, might a lot of that sediment around it that stops you from finding it, be flushed out so you can get to the pipe end?
