So my a/c gets low on Freon. Couldnt find a leak but when I charged it up I noticed when I pulled off the hose from the low port I noticed bubbles coming out of the low port valve. And its oily around the outside too. Its a slow leak I think. A/c stopped again so how hard is it to change out the Schrader valve on the a/c low port. Do I have to get my a/c system vacumed after I change the valve or will I be okay???