Lis,
The grey/silver lanyard connects to the "wet" or supply tank.
Referencing Spartans "Procedure for Draining Air System Tanks"...
Start engine, run at 1200 RPM until the air dryer purges at the back of the vehicle
Turn Coach off then back on to the "on" position. (gauges will function)
1st - Pull the grey/silver lanyard, (Wet Tank) - drain the tank completely, look at the gauges - they should NOT read zero. This indicates the check valve between the supply tank and primary tank is operating properly.
2nd - Pull the green lanyard, (Primary Tank) - check the gauges, rear air gauge SHOULD be reading zero and the front air gauge should NOT be reading zero. This indicates the check valve between the primary tank and secondary tanks are operating properly.
3rd - Pull the red cable, (Secondary Tank) - check the gauges, both front and rear gauges should be reading ZERO
Not following the procedure may have emptied the "Wet Tank" by the time you pulled the cord.
I tried attaching the 2 page procedure from Spartan but the file was too large, if you, or anyone who would like it, sends me an E-mail -
johnrugarber@gmail.com I can pass the information along.
I hope this was helpful!