What's up dex. Well first Ill say that I've read multiple places, and from my own experience, that runoff ph is basically useless. Sure it feels good thinking that the ph going in is the same coming out. My belief in this is that after bein watered with nutrients, the nutrients will begin to solidify. Over time this will change the ph, most of the time up if everything is working normal. However, I've noticed that when the ph goes down its due to salts.
Now most of the time my ph will drop late in flowering and I think this is because the plants will start to eat less and the salt concentration will become higher. Still nothing to worry about here as this is normal. What I want you to get out of this (and not saying you aren't doing it.) is that sufficient runoff is needed every time you water to wash out the old nutrients and salts.
Also, did you feed them full strength after you gave them a flush?
The ec reading of 0 is kind of weird. I really don't have an answer for that right now. All runoff should always have a reading of something because the coco should add some solids atleast. I'm a little occupied right now but when I get home I will try to get you a better answer.
Most of all you got to look at your plants, if you don't see them showing you any signs of being unhappy, then don't worry about it!! Can't fix what isn't broken, and those girls defiantly don't look broken to me. They look great!! :smokebuds: