Brakes - all of a sudden no stop

I've been driving this car with no issue for years.
Rebuilt years ago, stock sbp manual KH disc, rear drum, proportioning valve, I did upgrade to a newer style master cylinder, and a few years ago I rebuilt the calipers again for the heck of it.
Brake pedal has always been firm at the top, and gets better as it travels down.

I don't drive like a moron, always leave plenty of space between me and the person in front, however a few weeks ago I had to stop fast.

Since that moment I have a pedal that is weak with little resistance, from the top of travel to the bottom, and it will not stop.

No leaks whatsoever, calipers, wheel cylinders, lines, valve, mc, nothing.
I checked the fluid level, it is fine, I've actually haven't had the mc cap off in years and it never leaks.

Wtf happened here?

I don't want to throw parts at it, and I have some, I'd rather figure out wtf happened so I can address it, you know, as it's the brakes and all.

I was unable to locate a similar scenario, so any input is greatly appreciated.