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Oh - and yes I will remind myself to clean it before running it - as it does have oil on some surfaces that I am sure will get hot once...
I did find an 8 year old video and the bottom of the gauge appears to be fluid filled - which I am sure that similar to vehicle gauges...
Here is a pic
So quick easy question - and did small search with no luck. Got a decent deal on compressor via an auction. It was packed in original...