Greetings,
Doing a bit of looking around for another member who has issues with his grenades.
http://cgi.ebay.com/G-P-40mm-Airsoft...QQcmdZViewItem
Pretty standard slow leak, fills fine, valve is properly lubbed and seated, so I'm guessing the oring(s) is/are to blame.
Since my google-fu is a bit slow this morning, I'm leaving this here in case someone can point me towards the info before I find it.