Compressor completely drains from the switch box
Central Pneumatic Model 94667 pressure switch
I had my compressor run from nearly no pressure in the tank and I noticed that after the tank was full and motor stopped, there was a heavy leak that was near the tubing that goes to the on/off box.
The air wasn't coming from the fitting of the tube, rather near the bottom of the small plate the tube screws into.
I have no idea what this tube is other than you can see a contact point for when you toggle the on/off lever. I tried looking in the manual and it provides no help at all! See attached picture for the area I am talking about.
First visit the tank check valve and unloader valve pages on this site to understand what they are and how they work together.
Knowind that will help you understand what is likely happening on your 94667 air compressor, Chris.