Make sure the cylinder head is installed in the cylinder, put the complete piston assembly inside, including oring, put your thumb over the nozzle (the brass tube sticking out) and try to compress the piston. If the piston stops 1/4-1/3 of the way in, it's sealing properly.
Orings on the piston head seem oversized but they are meant to expand slightly during the compression stage to seal better.
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