I have been using my XT 29er wheels for awhile but put tubes in them. I at last got around to making them tubeless today and when I finally found the valves that came with them was puzzled with what to do with the little o-ring. One valve had it above the rim retaining nut, the other , below which looks more logical . I just did one wheel with the o-ring beneath the nut . Everything seated nicely but the thing keeps going down after 2-3 inflations. Is this an o-ring malfunction or is it just one of the many variables that seem to plague me when tubeless installing!

As far as I’m aware, the o ring is to keep water out of the rim (at least that’s what the magic ones are for). So yes, it should go under the nut. It shouldn’t have any affect at all on the tubeless setup. I forgot to install both of mine and haven’t had any problems.

Cheers for that, now how do you top up sealant fluid as the core doesn’t seem to come out of the valve? I ended up lifting a small section of bead and poked a small filler in there, not elegant but it worked.

In my experience, it’s generally better to add sealant exactly how u did it. Rather than through the valve core. I have found that the valves can get a bit clogged with sealant. I generally lift open a decent size section of tyre, and pour the sealant straight in. It’s easier if the bike is in a stand when u do it. Once the sealant is in, just spin the wheel so the open tyre is at the top, and close it up. Re inflate. Done.

P.s.: when I referred to “magic” valves in my previous post, I actually meant “mavic” valves. Stupid predictive txt!!!