Found this information online some time back when researching what to do about the mildew in my feeders.

"At least once a month, clean the feeder thoroughly with a solution of 1/4 CUP bleach to 1 GALLON of water. Soak the feeder in this solution for one hour, then clean with a bottle brush. Rinse well with running water and refill. Any remaining traces of bleach will be neutralized by reacting with the fresh syrup, and there's no need to air dry before refilling. Bleach is both safe and very effective."

Now this site says the bleach is not a problem, BUT like you I still have concerns about the bleach so after using the suggested bleach/water mixture to soak them, I always make sure to Rinse Rinse Rinse with hot water. Then I fill the feeder bottle full of hot water, add 2 drops of NON Concentrated IVORY dish soap, put on the lid and shake until suddsy. Then I let sit for about 5 minutes and then Rinse Rinse Rinse with hot water some more. I have several Birds that drink from my feeders, and they all seem to be happy and healthy.

Here is a link that might be useful: Hummingbirds.net

I've cleaned my feeders 1-2 times/week for years with vinegar, and my hummers seem to be healthy.

{{gwi:974319}}

Thank you Orca for the link and both Orca and Ctnchpr for the advise!
I'll try cleaning first with the vinegar, then with the bleach solution (I won't mix them!)
If the mildew doesn't clean out I'll buy a new feeder and be better about cleaning it. Nancy

It will clean out. With the vinegar you might have to let the feeder soak longer.

I use a bleach solution and rinse really really well...