Have used a good few 7mm Valprex valves, and only found one which wouldn't hold pressure.
Went looking for an O-ring a while back, as a slightly chubbier alternative to the rather thin (not much to compress) OE-size item that are supplied with Valprex valves. I had had one which leaked at the seal, despite inspection and cleaning.
Found this outfit...
http://www.polymax.co.uk/
who do a vast range of O-rings and other items, including Viton (FKM) rings...
http://www.polymax.co.uk/o-rings/rubber ... s178-viton
In this range, there is O-ring ref BS110V75, which has a cross-section of 2.62mm, rather than the OE/Valprex 2.00mm. ID and OD fit the valves fine, and there is a decent nip onto the ring when tightened. Normally use a smear of silcone grease on the ring as well, as it is a 'dry' joint.
The same O-ring will also fit Citroen OE sphere plugs.