Diana Bandit losing air

There's not much to this gun. It should have come with spare orings too. I'd first check the oring under the end cap. There's a ring pinned by a flat head screw. I had to tighten the screw on mine once. It holds down the fill port oring. Very simple design. Otherwise the end cap or valve body oring may be leaking. Or it is possible some tiny bit of debris may have made it down to the valve seal/seat it's-self 
 
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