John this is the way it should work. Try pushing it in with no air pressure it should pop right back out. Build pressure to 120 psi push it in shut the truck off push the brake pedal up and down when air pressure drops to around 60 psi the valve should pop out.
The valve is actually called a tractor production value. The operation of the valve is to apply the trucks spring brakes should the air pressure fall below safe operating range.
Please look up testing and operation of air brakes you should be familiar with there workings before operating any vehicles equipped with air brakes.
I was pretty sure you had a CDL if so revisit this portion of the testing if not you are most likely required to before operating the truck in question upon the highway. good luck have fun and stay safe.
