The evaporator is one of the multiple components of an air conditioning system. If you need to replace the evaporator in your Chevy Astro van, you are actually replacing the evaporator core located within the evaporator cases. On later models of the van (since 2002), you will not need to drain the engine coolant in order to remove the evaporator, but you will need to discharge the system and remove a number of other components to access it.
Take the Astro to an automotive repair shop with a licensed air conditioning technician who can properly discharge the air conditioning system. Bring the Astro home after the system is discharged and disconnect the negative battery cable.
Detach the overflow hose from the radiator's filler neck and remove the fasteners connecting the coolant and windshield washer reservoirs to the radiator core support. Disconnect the fluid hoses and electrical connectors from the washer pumps and remove both the coolant overflow and washer reservoirs.
Disconnect the electrical connectors to the heating/air conditioning unit.
Disconnect the inlet and outlet lines to the accumulator, using a backup wrench on the fitting to hold the line in place while you loosen the fitting with another wrench. Loosen the two bolts for the accumulator's brackets and remove the accumulator.
Remove the bracket for the blower motor relay and place it out of the way. Remove the bracket for the coolant bypass valve, located on the right side of the heating/AC unit.
Remove the clips and screws (using a screwdriver), securing the outer evaporator case. Separate the outer and inner cases and remove the evaporator core from the inner case.
Install the evaporator core into the inner evaporator case, and install the inner case into the outer case. Install the case into the engine compartment, using the clips and screws.
Reconnect the brackets for the coolant bypass valve and the blower motor relay.
Reinstall the accumulator with its brackets, and reconnect the inlet and outlet lines to the accumulator.
Reconnect the heating/AC unit's electrical connectors. Reinstall the windshield washer and coolant overflow reservoirs; reconnect the fluid hoses and electrical connectors to the washer pumps and the overflow hose to the radiator neck.
Have the air conditioning system evacuated and recharged, making sure the technician adds an ounce of refrigerant oil.