Red knob not popping out.

According to this publication, it is between 20 and 45 psi.

https://www.smartdrivetest.com/learning/testing-the-tractor-protection-system

 

Tractor protection valve is the valve that's mounted down on the frame or behind the cab. Its purpose is to protect tractor service air from escaping out the service line when bobtail.

Edit: The valve on the dash attached to the red knob is called the trailer supply valve. Modern trucks its usually integrated with the yellow tractor park brake valve.

 

I don't think it's broken. So it's kind of a moot point lol.

 

Its been a while. I knew it was somewhere around there.

 

Thank you all. AModelCat i have this valve in my truck. Will replace it tonight and post updates.
For those who arguing about OOS criteria a few years ago I was under DOT inspection and she did this test. Truck next to me was set OOS for exact what I'm saying.
Snapshot from Meritor manual will help

 

The very first paragraph contradicts #7 on that screenshot.

 

Ok i stand corrected.

 

I stand corrected.

 

Thank you, I was just about to say something but you saved me some typing.

 

Won’t argue with the manual on what it should be, but for OOS it is incorrect. For OOS the yellow button is not required to pop out and the red button must pop out before pressure drops below 20 psi. If she did the tests above she went well beyond her training or what is required.