low air warning from rear tank

Good afternoon
I'm getting a low air warning buzzer and light on my 2004 VNL 670. the forward air tank reads reliably between 105 and 125, but my rear tank gauge drops suddenly to between 30-45 psi. It eventually shoots back to normal reading around 120 at which point buzzer stops and STOP! warning light shuts off. These changes are happening to quickly for the air pressure to have actually changed in the tank, so I'm thinking there is a defective sensor involved, possibly at the air manifold near the transmission? Any one have this experience?

 

Hmm. Makes scense to me. Never got into the instrument cluster before but will give it a shot.
Thanks!