Diagnosing Air in Pipes

I've just moved into a house that uses well water. There was clearly air in the pipes, so I did some reading and bled the lines, pressure tank, and hot water heater. While the well pump was off, I noticed that air was hissing out of the pressure release on the backflow preventer or check valve located between the pump and the pressure tank.

Although I haven't located and inspected the well head and pump, I was told by the previous owner that an aeration system was installed at the well head to eliminate the naturally occurring smell of sulfur.

At this point I have a few theories on what could be happening, but I don't know how to proceed. While bleeding the water from the pipes did help for a little while, the air or gas has returned and our pipes are sputtering once again.

Any insights here would be greatly appreciated! Thanks!