You need to evacuate the system, replace the o ring, use a vacuum pump to remove any air and moisture afterwards and then recharge the system.
In other words take it to a shop that has the proper equipment to do the above.
96 T/A, Black/Graphite Leather, T-Tops, 6 speed, 10 speaker Delco/Bose "Monsoon"
Original owner, 197k miles and counting.
Total engine rebuild at 187,020 miles. AIR delete, bored .030 over.
Comp cams Xtreme Energy LT1 266HR-14 street cam and springs.
Custom leather interior at 190k miles. Total repaint 193k Miles