I would find the appropriate fitting to install a rubber hose and run that up and inside then body of the truck. You can put a t in the top, or simply make a loop so the open end is down. All the small fitting in the top of the master does is allow air in and out. You can find vent tips that hve small screens in them that are used in automotive applications.
If you look around, you should see a copper line running from the airpack, the big brake booster which is downstream from the master cylinder, to the air cleaner housing. That is normally where the air vent is tied into from the master cylinder.
I just did a complete brake rebuild on my truck, including new wheel cylinders and master cylinder and I rebuilt the air pack. I bled the truck myself without the "pressure bleeder" that some use and had no difficulties. I will keep tabs on this post and any help I can offer I am happy to do so.
RL