You might want to check your vent/vapor line that runs off your tank to the front of the car to the charcoal canister. It is most responsible for venting your tank. The gas cap is not normally operating in a vented mode. It has a spring loaded valve that will relieve pressure build up inside the tank if it gets hot to prevent expanding of the tank, and it will vent negative pressure (vacuum) if the tank vent line is constricted. If the tank vent line weren't constricted, the gas cap would not have to vent. So, I'd make sure the vent line isn't plugged up by blowing it out.
The test you did by blowing on the cap, is a test of the pressure relief, but not a test of its venting, although it probably wasn't venting, since replacing it made a difference. But depending on the cap to vent should not be necessary if the vent line is working as it should. So, that's something to consider, and you may not be getting as much line venting as you should be.
One of my cars started having the vacuum sound when removing the cap. It turned out that the front of the vent line end where it connects to the rubber hose going to the charcoal canister had formed some rust that plugged the end of the line. I cleaned it out, which solved the problem.
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Steve F.
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