It was my understanding that the old truck brake systems are vented and that they'll always go low after awhile. My '46 2 ton needs a top off once every 30 days or so weither it's sitting or not.. and there isn't any leaks, and stops on a dime.
Dunno.
I was at AutoZone the other day and in their bargian bin I found one way bleeder valves.. they let fluid and air out, but won't let air suck back in.. they were made by the same outfit that sells general use door handle cranks and replacement cigarette lighters and such.. I don't recall the name, but i've seen their products in auto parts stores coast to coast.. i'd bet you can find 'em anywhere.
Just make sure you bring one of your old bleeder valves in to match up the diameter and thread.
