It sounds like you still may be low on refrigerant. The system capacity is approx 28 oz. (1.75 lbs). In order for the compressor to engage via the Low Pressure Safety Switch, the refrigerant level needs to be at a minimum of about 0.5 lbs. If your compressor was not operating prior to adding, this would mean it was at or lower than this. So, if you added 2/3 of a 16 oz can, you approx added 10 oz, you'll need an additional 18 oz to make the system within specification. Do not overfill, it will trip the High Pressure Safety Switch, and casue the same concern. After adding the correct amount, your low pressure reading should stay within 20-41 psi, and the vent temp should be approx. 40-48 degrees. After adding, and running the a/c, monitor all the lines and connections that are visible, for any leaks. Your should here a "hissing" or notice liquid refrigerant leaking. It's abnormal to loose refrigerant unless there is some type of leak whether small or large, or if the vehicle a/c has been not used for long periods of time (5-7 years) the refrigerant can disapate under normal conditions.

-Rip