Most of the time aggressive cleaning is unnecessary, if the solution is changed before mold appears. Simply rinse well and brush with a suitably sized bottle brush. Pipe cleaners are perfect for ensuring the feeder ports are clean. Hummingbird bills deposit a lot of organic material in the ports and along with the solution are a breeding ground for mold, and often missed by the simple rinse and brush technique.
If mold has been visible in the solution or worse yet, can be seen adhering to parts of feeder more aggressive methods are necessary to ensure that the mold is killed off. I use a 50/50 white vinegar/water solution ! Soak the entire feeder for 10-15 min. then brush and rinse well. Always use this method prior to storing feeders for the season.
Heaven forbid you get a black mold that gives you the thought of tossing the feeder. Use a 25% solution of bleach/water, to disinfect and clean, instead of vinegar. Its the ultimate "solution" for feeders that have hidden areas that defy complete access, poorly designed feeders by the way. Many feeders are marketed for their aesthetic human eye appeal but little thought for the feeders functionality !!