How do you drain the air tanks on a commercial Volvo trash truck?

Under the door on the left side of the vehicle (I don't say "driver side" because Volvo does offer RHD models for parts of the world where that's the norm, and I don't know where you're posting from) is where the air tanks should be. The supply (wet) tank has a pull cord attached to it. Pull on it, and it'll drain the air out.