I just finished straightening out my brakes and the wrench sizes are 3/8 for the air vent on the master and 3/4 for the cap itself. I would recommend checking any trucks master cylinder daily until you know the idiosyncrasies of that particular truck. In my case, I had to rebuild a wheel cylinder and the master cylinder. Now the thing stops on a dime....sort of anyways...actually it has great brakes..
Just remember to refill the master after either each wheel cylinder you bleed, or else after two; no more....
. Ask me how I know that.....
also, a coleman fuel funnel works great as a means in which you can pour that gosh darn expensive Silicone brake fluid into the master. Just put it in the hole straight over the master.....