I decided to replace the vent caps with Siphon 360's. I have 4 vent caps. Could never figure out why 4. I've got 1 black tank and 2 grey. So by my logic I should have 3. But that's my logic, not Keystone's.
Anyway, I'm up on the roof undoing the caps and scraping up the old Dycor. Here's a pic.
As you can see, this pic of of 3 of the 4 vents. The 4th one is behind me.
Pulling up the 3rd vent, this is what I see.
Yup. That's right. No vent. If you zoom in on the pic you'll see a red circle. I assume that something poked a little hole in the roof material and instead just patching it, someone decided to add a vent cover.