If it's just the O-ring (as pictured below) then you simply need to put a new one in. Though you may have to go to the LDS to buy one if you don't have it handy (50 cents or so) As NetDoc stated that would make the air blow out the "pin-hole" on the side of the valve (not the back where the burst-disk is)
While DIN o-rings do not "go bad" frequently like yoke, they do occassionaly fall out and get lost (which would make sense if it's been in storage without a dustcap on) So it's always a good idea to have an extra (or 3) in your "save-a-dive" kit.
Aloha, Tim
P.S. if it's been sitting for 4 months, especially with the dustcap off, or in a warm location you may want to consider an annual servicing anyway...how long since the last "annual" checkup?
While DIN o-rings do not "go bad" frequently like yoke, they do occassionaly fall out and get lost (which would make sense if it's been in storage without a dustcap on) So it's always a good idea to have an extra (or 3) in your "save-a-dive" kit.
Aloha, Tim
P.S. if it's been sitting for 4 months, especially with the dustcap off, or in a warm location you may want to consider an annual servicing anyway...how long since the last "annual" checkup?