================= Check valve from compressor to primary tank, another from secondary to primary. Normally-closed solenoid valve from compressor to secondary, opens when primary >90PSI. Can be switched off. Primary feeds shop air line.
Compressor fills primary first to 90PSI, then solenoid valve to secondary opens, check valve into primary closes as compressor outlet pressure drops. Compressor brings up secondary.
If primary drops 90, compressor returns to filling secondary.
Normally secondary supplies airline via check valve into primary, is refilled through open solenoid valve.
When demand is too high and pressure drops below 90PSI, solenoid valve closes so compressor discharges into primary, as does secondary through the tank-tank check valve until the demand drops below compressor capacity. Then the compressor refills the primary, and then the secondary.
I think that covers everything. jsw