Does anyone know if there is an SMC fitting like a tee with a Schrader valve that I can merely splice into line #75 coming off the auxiliary compressor? I realize I can do brass, but a SMC would be simpler.
IMHO, a better point for the Schrader valve is on the line between the output of the compressor and the input of the water trap before the desiccant dryer. That way you run the air from the external supply through the dryer before it goes into line 75. You might need to install a small in-line check valve on the compressor side of the tee so the air through the Schrader valve doesn't go backwards through the compressor and exit the compressor air filter.
I think something like this fitting might work: Air Management - Fittings - Male Run Tee - Custom Connection (http://customconnectionllc.com/fittings/male-run-tee/)