Diana Air leak

I have a Diana Bandit in .22 cal. Haven't shot it in a while, so today I thought I'd shoot a bit. Problem is, it will not hold air. Fills OK, but as soon as I bleed off the compressor, the air cylinder bleeds down too. What do I need to do to correct the problem?
As always, thanks in advance
 
Classic description of a failure of the fill port O-ring.