We're getting a lot of air in our water supply from a private well. This seems to coincide with a change made to the well. Here are the details:

1. About 2 years ago, the well casing was shortened by about 3 feet, the pitless adapter was moved down the same 3 feet, and pump was lowered 3 feet. No length was taken off the pipe running from the pitless to the well pump. Well is about 80 feet deep.

2. I recently cut the main water line (from the well) to install a filter before the pressure tank. I thought I had the well pump turned off but didn't. Surprisingly, there was no water in the well line. After purging the main line of air, the house fixtures operated for about 2 days without any air in the lines, then it returned.

3. Today, I used the pressure purge valve on top of the filter to see if there was any air in it... nothing happened - no air or water came out... so I can assume that, as before, the water is filling the pressure tank and then backs out of the pipe - I'm guessing towards the well but don't know if it backs out all the way to the well as the response time to refill the pressure tank is pretty fast. When the well pump is running, the water filter will purge air at first and then puts out a steady stream of water.

4. When the well runs, there is a ton of water flow with no apparent air in the line - I get this from just observing the disconnected well line and the pump running.

So, it seems to me that there's a failed check valve that's causing the water to back up in the line until it's called for again. Or is this supposed to happen? Or is the pump sucking air as a result of having been lowered 3 feet? That doesn't make sense to me but I'm no expert on this...

Any help or testing ideas would be appreciated. Thanks!