My Sonex wing tanks have SAF-AIR push type fuel drain valves.
They leak or rather they weep very slowly.
The whole aircraft, including valves, have only done 185 hrs in about 2 years and the valves have been leaking for most of that time.
I make a point of pulling each one down after every preflight drain – doesn’t help.
The weep is so slow that this is more of an aesthetic problem – stains on my wings, than any practical issue.
All helpful suggestions welcome 
Hints, Fuel Lube - Dripping Fuel Drain Valve.pdf (992.6 KB)
just sharing hints article. I know it doesn’t take much “debris” to catch on the little o-rings to cause them not to seat properly. I would get new ones and swap them out, maybe draining the tank and doing a flush as well to clear any potential debris in the bottom. also, not sure if there is option to get them with viton o-rings?
technique wise, I usually try and take my samples in short bursts. thinking that the rush of fuel out would help “flush” any debris rather than it hanging up on the valve (o-ring)
Wing Tanks in Sonex? didn’t know that was a thing.
My project sat for a long time and the resilient element stuck to the seat, when I opened the valve it tore. They are not truly repairable; I renewed mine, and put a viton O-Ring in the old one as a spare, even though the sealing element isn’t actually an o-ring.
Thank you for all that.
Yes 2 X 30L wing tanks and a 40L fuselage header tank. Usable 97L
Hmm! I purchased the aircraft as an incomplete project, so I don’t actually know how old the drain valves are.
I completed the build, so do know how long the valves have been in use - not long at all.
I will investigate replacing O rings.
Thanks for your advice
Report:
Finally got round to doing the drain valves (part of 200hr service).
For those that suggested “stuff” in the valve, 
Valves had small flakes of paint between O ring & seat.
I had new Viton O rings ready to fit - looked thinner than originals, which appeared to be in good shape. Decided to give the OM’s another chance.
Cleaned & refitted - fingers crossed.
I always refuel through a (Mr) filter funnel, so no contamination from pump fuel. I think the paint flakes may have come from paint around the fuel filler cap/ports. Most of this has now gone, so future flakes less likly.