My Reba SLs are losing travel over the course of a ride – I think due to loss of pressure in the positive chamber. I’ve changed all o-rings in the air spring but still no change. What else is there? Is it likely to be the valve – I don’t have a spare Rockshox valve core – will any Schrader valve do the job?

Have you tried a valve key? My revs leaked over the course of 2 weeks or so, managed to tighten it up and they’ve been fine since.