A good vacuum pull here is a must! Do NOT skip this procedure at any costs!Moisture in the system will turn R134A corrosive and breathing this stuff will cause brain damage or even death at 4 parts per million and it has no odor at this concentration. Corrosion can eat away at the aluminum in your system, including the evaporator and this gas venting into the car would be lethal!

I'm not trying to scare you out of servicing your system here, but this is not an area where you should be taking any shortcuts. You want to really consider replacing the receiver/drier if your going to use R134A if that is what your AC compressor is designed for.