Christopher -
I can't find the info in Bentley either - however the valves are pretty much
always in close proximity (i.e. not more than about 1 meter or so) to the
compressor, which itself  is in the engine compartment. Just follow the two
pressure hoses from the compressor and you will find the valves for sure. In the
87 Westy one valve is perfectly hidden behind the rear-wall panel of the
cabinet which is in the rear left corner of the van. They have different sizes
to prevent errors whilst attaching the service gear to them.
Cheers
Chris

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