2016 W900 Brake lights staying on.....

This is good stuff too, much better than WD40….

 

Sorry for the rabbit hole I sent you down. Everything I said was based on the assumption that the gauge was seeing actual pressure.

Now I would think yes, you likely just have a brake pressure sensor? .

Never seen or heard of one, dealer should be able to tell.

Or, possible still, since it appears electronic driven, a relay backfeeding?

If you have a brake light relay, Swap it for another just to rule it out quickly.

That truck likely has a PDM on the left side of the firewall. I've seen 2 of those that had gotten extremely hot and melted terminals internally.

 

The reason I ask was thinking of potential blockage in the exhaust port.

Couple years ago my truck set at the shop for the week while I did some repairs. 1st trip out afterwards and I had an issue with the air application gauge slowly going down and noticed the brakes dragging. In my mind I felt a brake valve was going bad, but, the problem ended up being a plugged exhaust line! What time the truck set, those dang aggravating dirt bees had built a nest inside the brake exhaust line sticking through the firewall behind the engine. Chucked up a drill bit and evicted those rascals and problem was solved…..

 

"Normally", the majority of pressure sensors and switches are in the air manifold, not directly attached to the item it is related to.

But, I see you have two switches in that air line in the pic, so I'm not sure. I blamed bubba, but after a closer look, the yellow air line and first switch on it look factory.

 

Pull that traiiler stop relay just for S&G and see if it affects the gauge.

 

I've seen ice in air lines cause that same issue. Usually service side slider lines on van trailers. Water pools in the low spot and freezes, air can push past with apllication, but cant come back through to exhaust at the QR valve.