There's only a couple places a rapid will leak. Firstly bottle oring weeping from being screwed on and off, especially if it's not lightly lubed with silicone grease (very lightly - you don't want silicon grease getting into metal to metal contact points.)
Secondly reg oring failing, if its regged.
Finally the valve or valve stem seat into valve body.

If its been messed with to change calibre, it won't hurt to send it to kev G to be properly serviced and set up. He knows them inside out and is very very reasonable with what he charges when you weigh up parts and time he puts in.

They're easy to work on but this often means any monkey with tools won't mind having a go and getting it wrong.
Hope this helps,

Dan