On 15 Aug 2018 12:36:47 GMT, dpb wrote:

do the reservoir tanks "live" in a mostly topped-off condition
or on the bottom end?

I'm going to replace the pump gauge.
The water storage tanks are mostly in the almost full range.
We don't use much water. Probably 100 gallons a day or so (I'm guessing).

My plan is simple at the moment, now that I know how the thing works.
1. Replace the gauge at the booster pump
2. Determine at what pressure the motor turns on (and off)
3. Fill the bladder to 2 psi less than the motor turns on
(or, adjust the pressure the motor turns on - but maybe not)

Meanwhile...
A. Try to find a source for the entire pump + motor assembly, or,
B. Try to find a source for a parts diagram

Then...
a. Wait for the pump to not go on when it's supposed to go on, and,
b. Measure whether it's the pump or the switch that is failing.