AIR In A/C system

Had A/C charge last year with dye. Lasted a year (cannot find any leaks?) And now have the same problem. Also, when I turn on the A/C I hear a loud thump under the glove box. (Heard that the last time as well) All I get is warm air.
Thinking I just need a charge like the last time, I went to Auto Zone to buy R134. Auto Zone told me I had air in my A/C system. He then told me to hold down the valve inside the low port to release it. He actually did that for a few seconds to show me how to do it. It sounded like he was letting air out of a tire. I am reading that it should be vacuumed and have gauges? I am now doubting his advice.