OK...I thought I seen just about everything when this morning I'm bleeding the brakes and I develop a leak in the filler cap of the master cylinder. not around the edge but in the casting at the point where the nut and flange cap meet....crazy. I'm guessing there may be a hairline crack in the casting or it has become porous enough after 74 years it leaks....
Hy Mr. Crosswrench, the old hydraulic brake systems were vented, usually by a small hole drilled sideways where the square or hex meets the flange, hope that helps. Please remember to include the year, make and size of truck you're inquiring about.
