Glad hand question

[998Junkie]

I've been working on my truck and I noticed something that has been playing with my head all weekend, so maybe someone can make some sense of it.

On the front of my M818 are the towing glad hands (Emergency and service). The emergency has a shutoff valve but the service one does not. In addition the service glad hand cover has a breather cap on it and a hole has been drilled into the cover. This allows air to escape. To me it seems strange that the glad hand for the service brake allows air to escape, when I look at my air pressure gauge it appears to be reading 100 - 120 psi and when I hit the brakes I don't see the gauge move, so I can't seem to figure out why this modification?

Has anyone seen a mod like this? Is it there to keep the system from over presurizing?

 

[cranetruck]

This is the front "service" glad hand only and it has check valves further down stream, if pressure builds up due to any leakage through these check valves, it will lock the brakes up, therefore when a glad hand cover is used, it must be vented.