Ok, some experimental results to report. I removed the rubber gadget from the lid, cleaned it and all the mating surfaces, both on the lid and on the MC rim, reinstalled with the rubber pointing up, and down, no difference, still leaked from the fluid sloshing around while driving.
The gasket isn't 45 years old btw @Mustangman , truck isn't quite that decrepit, lol , the master cylinder ass'y was replaced 3 years ago.
From everything I can see it is just fluid sloshing over the top of the MC, somehow getting by the gasket. Which seems nearly impossible. It looks to be mostly leaking at the front edge, the edge furthest from the firewall. Not entirely surprising since the MC is tilted slightly downward, maybe 2-3 degrees, so the fluid will be more likely to slosh over there. Also that front section is for the rear brakes I think so it has a smaller reservoir than the back part for the front brakes.
I tried pressing on the brake pedal numerous times and vigorously while the truck was stopped and engine off, did this before the drive test, and no signs of leaks occurring just from pressing on the brake pedal. Almost certainly from the truck's motion.
I think the theory about the cap being bent is possible. It was probably a rebuilt unit that I purchased as a replacement. I'll take a straight edge to the cap and the top surface of the MC, see if I can determine anything. It didn't leak at first though. I think I still have the old cap from the failed MC from 3 years ago, so will try that too.
I did notice when cleaning the cap there's two tiny holes at the top, but I think those are supposed to be there, to allow air in as the fluid level drops from brake use. In any event the leak isn't coming from those holes b/c the rubber gasket is in the way and no signs of brake fluid getting into that area.
Also thinking of using some thin gasket material and making a gasket to exactly fit the rim of the MC. that would be a good opportunity to also test the theories in the other thread how to make flimsy gaskets stay put.