so u have the AC on when u checked the gauge on the can?
i did this to mine last week my steps to a full recharged system was:
1. attach r-134a to low connection ( can should read overfilled until u turn on AC)
2. turn on car and have the Ac engaged (can should read zero or empty now)
3. have some one inside give a bit of throttle as you untighten the screw so the freon will come out. Person inside should say that they feel the cooler air. and when the can is empty or at the safe level on the gauge youre done.
just lettin ya know how i did it.