RE: Reaction ARF - PST/BTE All-Composite Sport Jet
Kev,
I don't know the size... but I did have the same problem you have (Both on a BVM and PST brake unit), to fix it I didn't replace the o-rings. I slathered O-ring lube on the rings and now it's as tight (airtight) as the day it was born.
Todd has the stuff.