When you ask about brown stuff leaking out your valve hole area you are bound to get some amusing - or otherwise - responses. 
My bet brian, is still that its something to do with the stuff the tyrefitters wipe around your rim before easing it on. oooeerr missus!
I had two tyres swapped over yesterday and the fitter plastered plenty of oily coloured liquid around mine before .... ah, bother, there we go again! I'll get me coat. 
One of mine had a very slow leak caused by crud/corrosion around the wheel rim which caused very slow deflation. Have you monitored the pressure over a week or so?
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2002 D2 S8 – Ming Blue, Valcona leather, Vavona wood insert, solar sunroof (to be fitted), Heated rear seats, extended leather pack, 18” Avus, ski hatch, Bose, auto dim rear view mirror, rear blind.
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