I have a different take, based on your carefully worded symptom.

My guess is you have a dragging caliper. A sticky caliper. In the extreme case you will lose brakes altogether when this occurs in a front wheel after freeway cruising. The extreme case will occur when the old brake fluid, with its reduced boiling point from the original DOT-4 value, boils in that sticky caliper and boils in BOTH circuits.

For now, it is just creating compressible water vapor in one circuit, or to the extent you DO reach the pedal bottom when you get to your toll plaza or off ramp stop. Until you find this issue, keep that handbrake accessible, and take care when exiting the freeway.

Yes, the master is probably due for replacement. This is because the boiling fluid has allowed its travel to send the seals into the rusty, unused areas ahead. But it won't cure the issue. And, in my opinion, you can forget the booster as a cause. Derek's observation that loose wheel bearings can increase pedal travel (pad knock-back) is true enough, but you wouldn't have to talk about freeway driving for that to occur.

If the master alone would be the cause, it would do it in town, and within the first few minutes of driving. I interpret your symptom report to say this happens after high speed driving for some time, which is consistent with a sticky caliper and dragging pads. Check your wheel temperature after a high speed drive. If you don't have a non-contact IR thermometer, use your nose and the back of your hand near the wheel.
--
Art Benstein near Baltimore

A cowboy from Texas was pulled over by an Arizona DPS Trooper for speeding.

The trooper started to lecture the cowboy about his speeding, and in general began to throw his weight around to try to make the cowboy feel uncomfortable.

Finally, the trooper got around to writing out the ticket. As he was doing that, he kept swatting at some flies that were buzzing around his head. The cowboy said, "Y'all havin' some problem with circle flies?"

The trooper stopped writing the ticket and said, "Well yeah, if that's what they're called. But I never heard of no circle flies.

"Well, sir," the cowboy replied, "circle flies hang around ranches. They're called circle flies because they're almost always found circling around the back end of a horse."

The trooper said, "Oh," and went back to writing the ticket. But, a moment later he stopped and said, "Are you calling me a horse's ass?"

"No, sir," the cowboy replied, "I have too much respect for law enforcement to call y'all a horse's ass."

"That's a good thing," the trooper said and went back to writing the ticket.
After a long pause, the cowboy, in his best Texas drawl said, "Hard to fool them flies though."