The o-ring you need for your vacuum breaker is OR102 - but don't get too excited yet. OR102 is from an Italian specification which has evidently(?) been overtaken by other specs. I BELIEVE the proper o-ring for this application has a width of 1.5 mm, an OD of 7 mm and an ID of 3 mm and material = silicon.

LM shows it as OR102 in their Linea parts and Synesso talks about it in their troubleshooting manual. Gary was being kind when he said "It is a surprisingly confusing market place for o-rings - lots of different designations across metric and american sizes" and Greg was correct to reference the Parker catalog. Here is another listing to look at: http://o-ring.info/en/o-ring/Oring-Handbook/ERIKS_SealingElements_TechnicalHandbook_O-rings_SizeCharts.pdf .

You are going to waste enough time (and possibly money) by trying to fix this problem that you are so much better off by simply buying a replacement valve and then soaking your existing valve in white vinegar for about 30 minutes. But, if you insist , McMaster-Carr has your o-ring - it's just a question of coming up with the absolutely correct size.

is this the right part # 5233T46 from mcmaster-carr

OK - I had a math error - my apologies. Maybe your best bet would be to take the o-ring to an industrial supply house as previously recommended.