>Essentially after that abrupt stop, the brake pedal went to the floor
>and I had to pump the brake pedal to get brakes (I used to do that in
>my old beetle, so I adapted with little trouble.)

This sounds like a blown rear brake seal...

>Later (many hours later on a separate drive), the brakes returned to
>normal, i.e. a stomp on the pedal felt normal (no pumping needed) and
>braking action occurred as expected.

... but this doesn't!