Trailer disconnect switch.

That's my point. If it's supplying the red line it's not for a jaw release. The jaw release is it's own hose to the 5th wheel, just like a slide release.

 

Don’t know if this is any help, but I have one also.
I was told that this will help the trailer brakes hold when trying to move stuck trailer tandems.

 

If it was a fifth wheel release, I can see it dumping the emergency line when you hit it, but not charging it.
It would be easy enough to look and see if you have an air release on your fifth wheel.

 

It's primary function is for vehicle inspections - check for leaks and lights.

If you trailer spring brakes really suck it can help when sliding tandems, but it's rare for me to need that.

 

You put a dumb gladhand on it so it's supplying air to essentially a plug. It only THINKS there is a trailer on there.

It's been years since I had a three button tractor. But I aint forgot.

 

Once the red trailer supply line is pushed in, the trailer hold cancels.

The story I heard is large companies were tired of drivers mis-using/abusing the trolley valve, but still needed a way to check brake lights. A switch was created to replace the trolley valve with a protection feature to make sure drivers don't drive down the road with the trailer brakes gently on.

 

It has nothing to do with the old 3 button systems, it is on trucks with the abs brakes, and yes I started out using dummy glad hands, when I was doing some power only with quite a bit of bobtailing.