There was a post on here recently about changing the seals on poppet's but mine are a funny shape, not like a normal o-ring. Check your out, hop & grape sell poppet's if you need a new one
a poppet leak can generally be confirmed with a damp finger over the poppet top and can usually be fixed with a little re-seating with the back of a tspoon (can be messy tho so do it outside..)