Picked up a Yellow Jacket 4 oz Oil/Dye Injector at an auction. Can someone explain how to use it. It has a small hose and valve on one end and a schrader valve on the other end. It is obvious you screw off the end with the scrader to add the oil but it appears any way you use it will inject air into the system. Only thing I have found so far is you can push the oil with refrigerant or draw it in with a vacuum but how do you purge the air out of the injector once you add the oil to the tool and replace the cap.