Exactly. Air compressor tanks contain a bit of water which builds up over time because of the pressure, humidity, etc. When I used to maintain our internet cafe before, I had to get a water filter -which looks like a motorcycle engine's fuel filter- clamped between the valve and the hose to hopefully separate the water from the air before I clean the cpu's.
There were still tiny bits of water coming out so imagine if it didn't have that water filter.





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