I have a Dura Ace Scandium wheelset on which I am running Hutchinson Tubeless Tires. I have had trouble with the rear tire losing pressure while the front tire seems to be normal. By normal, I check prior to each ride and may or may not need to top up the front to the 95psi I like. The rear wheel will take 30-40 PSI everyday and after 3 days will be almost flat, say down to 10psi.  

This was worse when I first starting using the tires. I have taken the rear apart and reinstalled several times to no avail. The last time I reinstalled I double-checked the valve. On the DA's the valve stem is offset a bit and there is a valve stem holder that is screwed on which has an angled cut to it so that it sits flush with the edges of the offset hole on the rim. I always thought this was a weakness in the design and that the seal was not ideal, allowing air to leak. I even tried adding a narrow O ring under it to seal it off more. This made it worse.

I then turned my attention to the valve itself. Because there is a bit of offset the valve has a tendency to twist when everything is tightened down. I held the valve with a set of needle nose pliers as I tightened down the seating parts. This has helped a little as I now only lose about 10-15 psi per day on the rear wheel.

Any other suggestions or is this as good as it gets? I haven't  used slime products because I'm trying to keep this light for training and as close to race weight as possible.

Over tightening the tubeless valve stem retainer is the cause of most Tubeless leak-downs. Less torque is best for that seal (go figure). Your problem may be more simple – air passes through the walls of tubeless tires a bit faster than it does through a tube and a tire pressed tightly against each other. All lightweight tubeless tires lose air at some rate unless you add a liquid sealant like Stan's or CaffeLatex (my two favorites these days). Try some sealant and, my guess is that your air loss problems will stop.

Contact Richard Cunningham for questions or comments, or just to talk bikes at: askRC@roadbikeaction.com