[MARKGMC3]

Here is the schematic of the system & shows how it ties into the main air supply system. 

Attached is the 12V air system and how it interacts with the main air system.  The 12V air pump only puts out 60 psi.  The main air system puts out 120 psi.

To make things easier start with the main air system having no air pressure.  Follow the air from the 12V air pump and you'll see how it pressurizes both slide seal regulators, both of my toilets, both pockets doors and the 3 gallon air tank in the right wheel well to 60 psi.  Note how the check valve on "T" #3 on the air tank will not allow air to exit into the main supply system.  

Now energize the main air system with 120 psi of air.  The stronger air is supplied from the stubbed off main line and energizes both side dump valve solenoids with 120 psi.  The stronger air can now enter the 3 gallon air tank, through the check valve, and boost its pressure to 120 psi.  120 psi air now goes to both slide seal regulators.  The stronger air enters the 12V air pump four way and tells the pressure switch that there is more than 60 psi so no reason to turn the pump on.  The 120 psi air goes through the 12V air pump regulator & is reduced to 60 psi and supplies both toilets, and both pocket doors with 60 psi air.

If any one wants a copy they can print send me an email.

Yes..I would like a copy.. [email protected]

Thank u.....hope it is the same on my 1999

Mark