I've just stripped my EWL beer engine down to see what might be causing beer to flow back down the pipe after each pull. When I removed the bottom part of the cylinder, I found that the non-return valve wasn't seating properly and it looks like this might be the culprit:

IMG_0412 by Paul Mead, on Flickr
I'm assuming that the o-ring is supposed to fit snugly in the groove, is this right? Any idea where I can get the right one?
Thanks