When should red trailer valve pop out?

Your buddies dump trucks probably have glad hands on the frame like this for the trailer to hook to... They have shut-off valves built into them.



If he was to open those valves with no trailer attached... Then his red valve wouldnt stay pushed in either.

Straight trucks and dump trucks are different than tractors.

 

I didn't say to push in the red valve while bobtailing. I said during the brake check procedure that should be part of your daily pretrip procedure.
Without the supply line connected to a trailer and the valve pushed in, you're losing air. The tractor protection shuts down around 30 psi, the knop pops out.