Questions about low air

[curlesw]

We were on the last few miles of our 4,000 mile trip yesterday and
home was just a few miles down the road when my "low air" light and
buzzer decided to go off. We had just finished descending a 4,100+
mountain and all my gauges were perfect. I immediately checked all
three air gauges and all three were well within the normal operating
limits. I checked my brakes and all was fine but nervous given the
situation. I continued slowly on and watched as the system increased
pressure as it normally does once it gets too low.

We were able to make it home just fine but the light and buzzer never
went off, even when the system was clearly operating ok (i.e. the
gauges indicating all was well). Once home, I turned the Bird off
and walked around listening for leaks but could not find/hear
anything. Today, I turned the Bird on and light and buzzer went off
once the system was fully pressurized. I drove to storage and all
was normal, go figure. As such, I am thoroughly confused and have a
couple of questions.

1) Has anyone experienced a similar problem?

2) How long should a properly functioning system stay pressurized?
After 12 hours of sitting, my system will require a few moments to
fully pressurize once I start the Bird. Also, when driving on the
highway for example, the system goes from full to low and then pump
back up to full every 20-30 minutes. Is this normal?

Thanks.

Wayne C.
San Diego, CA
1985 PT-40