How do you clean an evaporator drain line in a home ac unit?

Hook a Wet/Dry vac (with the filter removed) to the outside drain line. This will allow you to suck the drain line clean and remove any water/algae/debris from the line. As a maintenance item, you may want to pour 1/2 c of household bleach into the drain line downstream of the drip pan to minimize the future growth of algae. Do both of these items yearly to keep problems from recurring.