As Randall noted, it HAS to be getting past the O-ring. Either it's the wrong size or been compromised in some way.
This may seem silly on the surface of it, but: if the bore is burred, try using a bamboo chopstick, hacksawn with a slit and some 600 grit wet-or-dry... like honing a cylinder wall. Some light oil and patience. :wink:
<span style="font-style: italic">EDIT: Don't be tempted to use a drill. Do it by hand.</span>