I'm assuming you've already done the normal sand clear outs on the intake valve?
With the airport valve, the screw cap is supposed to be the air tight seal - not the twist flap inside. That's just to make it leak less whilst you close the screw cap. Check that the gasket inside the cap is A) still there and B) not damaged. Maybe try flipping it over in case its a bit scuffed up. Might save you a long trip if its just some grit/sand stuck in the cap somewhere?
If its not leaking from the screw cap then its a different story. Those valves should be ultrasonically welded onto the bladder and its a case of cut them off to replace. I'd be seeing how much good will you have with your shop to see if they can get it sorted for you on something that new. The more testing to work out exactly where its leaking you can do prior to going to the shop, the easier you can make it for the guys in the shop to help out there and then.