To properly discharge and charge the A/C system a special charging system, quick connectors, complete training in refrigerant recycling/special service procedures, and a certification license is absolutely necessary. DO NOT VENT ANY AMOUNT (EVEN SMALL AMOUNTS-UNDER PENALTY OF LAW) OF REFRIGERANT INTO THE ATMOSPHERE.
If you are not properly trained and certified, take the vehicle to a certified mechanic in order to have the system evacuated and discharged. Immediately after finishing work on the vehicle, take the truck back to the mechanic to have the system recharged as soon as possible.