That grey little plug is where my old Sense and Dispense system put water back into the system. I've covered it with Saran wrap too for the short termWhat about that gray little plug in the black fitting going directly into your pump basket? Have you tried putting Teflon tape and lube on that?
Your PVC connections all look good from the pics, but air leaks are tiny little things that are very hard to find (in my experience). I fixed an air leak I had, but never pinpointed exactly where the problem was...so now I am back to square one troubleshooting again.
I don't see any unions in your pics, but if you have unions on the suction side take them apart and inspect the o rings inside. Make sure no cracks or wear and tear, lube them up good, and tighten unions back together.
A good tool for loosening unions is an oil filter wrench if you don't have a rubber union wrench. A lot of people prefer the oil filter wrenches actually, which is what I use.