I had a slow pressure loss on three tyres earlier in the year
. It turned out that the retaining nut on the valve on my tubeless wire wheels had become very slightly undone (not unheard of apparently). This means that the internal sealing ring on the valve (located internally) is no longer pulled tight against the inner surface of the rim to provide an air-tight seal. After confirming this was the source of the leak with a weak soap solution all that was needed was to slightly tighten the retaining nut (external at the bottom of the valve adjacent to rim) and all was well and has been since (about 6 months)
.
Hope this is the easy fix for you too.
