Ignoramus31823 fired this volley in
:

Think some more, you will realize that what I want to do is the nicest
way of doing it.


If your pump cuts in at 90, then all you've done is complicate the system
without improving the performance. The INSTANT your system hits 120,
it'll dump _all_ your main tank capacity into the reserve until the
pressure drops to 90 -- then the electric valve turns off and the pump
starts to run again... it will not improve the length of time it takes to
get the reserve up to pressure. That's dependent entirely on pump
capacity. It matters not how you fill it, it will always take the same
length of time.

Your method will NOT improve run time, or the time to reach "usable" air
pressure in the main tank, except for a slight (say 10%) time. But it
will increase the complexity and maintenance issues over a simpler,
purely solution.

Ig... for any given solution that will work, simpler and less expensive
is always better.

Lloyd