Pressure falls after pump turns off, but levels out

[Reach4]

My pressure switch is set up for about 37/57. I had noticed some predictable drop after the pump had turned off with no use. I set up a camera on my mechanical and digital pressure gauges. At a time that the pressure gauge was about 41, I manually tripped the pressure switch and recorded a video. No water was consumed for many minutes and during the test.

There was quite a bit of pressure gyrations in the first couple seconds, but it smoothed out. The mechanical gauge had the most bounce. This is a graph of the reading from the digital gauge with time zero normalized to the second that the pressure switch clicked off. The video recorded at 30 FPS, but this graph just gives the pressure once per second.

While I did not graph it, the video continues for 36 minutes 26 seconds (2129 seconds) when the pressure read 54.16 PSI.

So what is going on? Here are my candidates:

1. The check valve at the pump takes a while to seal completely. It is odd it would be so consistent.
2. The pressure tank diaphragm and, less so, the schedule 80 drop pipe and 25 ft long polyethylene tubing,have an elastic creep? That sounds odd. The diaphragm should be more like a bellows than a balloon, right?

This graph looks similar to a capacitor discharge.
What do you think is happening? Is this common?

 

[Valveman]

A pressure relief valve will sometimes act like that, but you should notice water coming out of it. So it is probably a bad seat in a check valve. I doubt it is the muffler bearing.