Leaking Air When Pressing Brake Pedal

Your welcome.

You already did test it. The valve is stuck open.

 

The response given by the quote below by @Goodysnap would be the answer. I have not read everyone else's response but the gentleman that quoted below me is a full-time mechanic and he knows what these problems are. He is a very valued and trusted member on the site regarding mechanical issues. I would listen to whatever he tells you.

 

Last time I had an air leak from the exhaust port on the manifold it was the handbrake lever valve. But that was on a euro truck that only has 1 handbrake (not yellow and red)

Usually the exhaust port through the firewall is for when the handbrake valve isn't working properly.

So if your red button is out when driving and you are still getting air through the blue air line, I would be looking at the handbrake valve on the dash to start.