PVC union help - leaking

[Delphinus]

Could be as simple as a grain of sand on the O-ring .. doesn't take much. Check for nicks or abrasions too. Worst case scenario, you can replace the O-ring. Home Depot, etc. carry various O-ring sizes but if you're really in a bind and can't find the right size, get one that's too big, cut it size and butt-end the cut ends together and glue together with superglue or frag glue. Should do the trick...