Air Valve leak? Peterbilt 579

Hello Everyone,

A truck that I do not own but after pushing in the yellow knob i heard a leak from the valve on dash. Applying more pressure i heard a leak from driver side corner of engine. I opened the hood got someone the press service brake and found where the air was coming out of (i attached a pic and circled in red). Some mechanic at my company said it was a plug missing and shoved in a plug. Of course the leak stopped from there but now when you push in yellow knob, it drains air very quick. Also hear hissing sound from yellow truck brake knob when pushed in. I also hear it when its pulled out and apply the service brake hard. Maxi leak can be an issue but I didn’t hear any leaks at the back.

i attached a pic second image is where he put in the plug. The red thing but that didnt work and i think he should not have done that? Not sure. Anyone else experience this in the past?

peterbilt 579 heavy spec 2018 auto.