Ok this morning the pressure was at 0 psi. Because I had switched the breaker off, I don't know if the pump would have kicked on at 20 psi and been able to re-pressurize the system or not.
As there are no leaks in the pipes, joints etc. the only thing I can think of is leaks through the check valves back into the well points.
I had to re-prime the system, and even at that it took 15 - 20 minutes to build enough pressure to properly work the sprinkler system. You could hear at the sprinkler valve that there was a lot of air being pushed through the line. My thought is that there are 9 well points and the water is 8' below surface, so there is a lot of air to move before the pump really can get up to normal operating pressure / volume.
Is that a valid assumption?
After that, all seemed to go well (no pun intended). I ran it through an abbreviated sprinkler schedule and when the cycle ended the pump built up to 40 psi and cut off as it was supposed to. Pressure stabilized at 36 psi again.
Leak down by the hour has been as follows:
1 - 36 psi
2 - 36 psi
3 - 35 psi
4 - 35 psi
5 - 35 psi
6 - 35 psi
7 - 35 psi
8 - 35 psi
9 - 35 psi
Based on the 0 psi this morning, this is rather unexpected. The pump has not kicked on at all, so the pressure has been steady. I'm going to monitor for another few hours and see what happens.
So what should I see happen if the system is functioning the way it should?