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On Fri, 06 Dec 2013 19:47:58 -0600, Ignoramus5722

Here is what I would use. The 60cfm model.

http://www.airtekltd.com/valves.htm#LOAD_GENIE

It will open and allow flow to the secondary tank at 120psi and
block
the flow when the primary tank drops to 90psi. A parallel line with
a
check valve to allow flow from the secondary to the primary would
allow both tanks to supply the system below 90psi.

I've had a Load Genie on my compressor since the 70's. It tends to jam
from sideways loading on the inlet pipe, which expands when it heats
up. Sometimes the Load Genie waits a minute or two after the
compressor shuts off before unloading.
jsw