Air leak at the compressor / governor

My truck started makin a hissing sound / air leak when I shut it off. I didn't remember hearing it before so I tracked it down to what I believe is the governor on the air compressor. The small button looking device to the right of the hoses is leaking and then slowly shuts off. It doesn't seem normal but I'm not sure what the function of this device is. It looks like a bleeder to release pressure but the cap is loose and just sitting on top. Anybody familiar with this button / cap ?

 

Thanks for your reply.
Yes the is a Schrader valve under the red cap cover but if you look to the far right you can see another item that looks like a relief valve vent or bleeder.
Im wondering what it is and why it is leaking air pressure - is this normal and I am not to worry or if its a issue waiting to be a problem?

Any help on this appreciated.