I would check your air supply you may have a check valve in it
This is used on compression gauges
lots of people use the fitting to fit the spark plug hole and have pipe fitting on the other end to hook to the air supply
The problem arises when the compession tool is made to let air out of cylinder
not in
Then their is a relieve button to drop the air out
Check to make sure you have air coming out of the fitting that goes into the
spark plug hole
Seen this happen
Easy misake when in a rush to get done

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[B]Mike Y.
00 sl500 sport
88 560 sl (sold)