An RT air leak out of the tiny pin hole on the side of the valve is from your regulator pin. Reg Valve Pin Assembly - Retro
First take it out and look at it. If it looks brown and crappy it might be bad. If the orings look good then try oiling it well, re-install, then dry fire a bunch to see if the leak goes away. If it still persists, then it either needs to be replaced completely, or if the tiny spring pack on it is in good condition still, then you might be able to just replace the two tiny little orings at the end of it and be good. The two o-rings have a plastic spacer oring between them thats split on one side so you can remove it as well, but you shouldn't have to replace it. Lube generously and re-install the reg pin. Once you air up, it could take alot of dry firing the gun to get the reg pin to seat properly with the new orings, or break in a little if its new, and for that leak to dissapear.
This is an expensive little part to replace, but a good one well maintained should last for years.