Bendix AD-IP Air Dryer question..

From what I read, the AD-SP does use system PSI to purge....so I thought a drop in PSI would be expected...?

 

Yea, sorry about that....my mistake.

I see a loss in the FRONT system (green needle) , not the REAR system (orange needle) and from what I can read
thats normal for the AD-SP.

 

This makes sense....but the gauge is labeled GREEN (Front) and ORANGE (Rear) and anyone else with our setup sees the green needle fall after AD-SP purge.....so while I never really understood what is what, my setup appear normal.

Bendix claims that the AD-SP will draw from the FRONT tank and per the gauge, thats whats going on.
Again, not that I understand this or dispute what you say......

 

I've been thinking about this still......
HeavyD looks like he is right....

"During AD-SP Purge, System pressure will be reduced in the supply (wet tank) and secondary (front axle) reservoirs only. The primary or rear axle service reservoir will retain full governor cutout pressure.

I see the two needles at 120 then the purge and the ORANGE needle stays put and the GREEN needle falls.

Ok..with that, I understand the following:

GREEN circuit = PRIMARY AIR
ORANGE circuit = SECONDARY AIR

The gauge on the dash has 2 needles (Green and Orange) but they are labeled:

Front
Rear

If they said PRIMARY and SECONDARY it would make this much easier.
Which needle SHOULD fall when AD-SP purges?

Adding insult to injury, a year ago this gauge was replaced. I wonder if it was connected properly!?
I have three tanks on the W900:

I believe the wet tank is the small "pig" looking one and then there is one large tank directly after that and then one
large tank under the driver. I do not know which of those tanks is the primary or secondary.