I do not flush BUT once a week I serve my plants a tea of beneficial organic stuff such as Liquid Karma, Molasses, and other low PPM natural stuff.
I would check your run off.
If your runoff comes out what you are pouring in no need for a flush
What medium are you using?
what Nutrients?
Also I have never ever seen brown leaves from overwatering.
Signs of overwatering are the tips of the plants making a 90 degree turn and pointing to the floor.
If you are see signs of brown leaves starting from the tip of the leaf and working its way down to the stem it could be nute burn from feeding too hot.
Just giving you some advice from issues that have happened to me