Master Cylinder Upgrade

[dstrausb1]

Hi Guys.
I'm preparing to replace my leaking master cylinder with the newer dual circuit one that has been described on the Forum in the past. First thing I notice is that, coming off the top of the master cylinder, there is a petcock (which is open) with a brake line leading over the frame and towards the front of the truck. None of the posts I've ever read on master cylinder changes has mentioned this line or what to do with it.
With luck, I've attached a picture taken from underneath the truck. You can see the petcock just below the access cover in the floor with the line curving behind the brake light wires.
Can anyone tell me what it is for and how to include or exclude it during the conversion?
Thanks,
Dan

P.S. I guess I can't attach the picture after all, I just got this: "Sorry, the board attachment quota has been reached.".
Any help will be appreciated.

[Rick C]

That vents the master cylinder. It goes to the air cleaner horn I believe.

[snowdad]

Rick is correct. It helps in fording.
Phillip

[dstrausb1]

Thanks for the guidance guys, I'm self propelled again.
Dan.