The canister is charged with freon and co2 and the quick connect has a needle valve that allows the freon and co2 gas to be released once connected otherwise once you unscrew the canister any remaining gases that was put into the system will be released so its very important to make sure that the canister is completely empty before removing the hose from the canister. Hope this answers your question and thanks for the question
That whole setup for the can and holes was only 35-40.00 bucks and you can get a Is temperature gauge or even the multimeter with that attachment on Amazon for 20-30.00 dollars or just use the little temperature indicators that come with it, it should cost you more than 50.00 dollars to recharge your cars ac unless it's leaking or there's more damage than just needing to be recharged
