Not the Autovac Again

About three months ago I thought I had the leaking Autovac stopcock problem solved, using rubber as the material for the sealing washer. It was uncertain whether that rubber would be resistant to attack from diesel oil: it isn't. The thing is leaking again and the rubber edge leaves black on your finger tip.

The tip of rubber can be seen poking out of the circular body of the three-way cock on the right. Unfortunately I didn't make the camera focus on the stop-cock: the engine cover took precedence. But if you click on the photo you can just see that on the hexagonal mounting flange above it says "OFF", and "RESERVE" to the right. On the face to the left out of sight it says "RUN". That is what the cock is for: to provide a reserve supply if you run out of fuel on the road. Seeing that all Government buses were refuelled without fail at the end of every shift, and had 35 or 40 gallon fuel tanks, it was a feature unlikely ever to be used.
Next try: use rubber from a piece of an old tyre inner tube, cadged from the tyre service. This should be butyl rubber and oil-resistant. If not, high quality cork sheeting may have to be resorted to: the original was this material but it's not easy to find nowadays.
The Autovac is such an integral part of Leylands of the mid-1930s and the three-way cock so inseparable from it, that whatever the problem it will be overcome.