The Nissan Path

I have a 2004 Pathfinder that I use infrequently as a tow vehicle. The last time I used her the brakes were fine for the country road part of the trip but after driving on the highway and not using the brakes for 20 miles, the pedal went to the floor as I approached the exit. I gave them a couple pumps and they were fine. As long as they are used every now and again they work great. If not used they lose pressure. When driven around town, you would never know there is a problem. There is steady pressure within the first 1/2 inch of depressing the pedal.

Pads and rotors are less than 5000 miles old. I bled them again with a vacuum pump. Brake fluid is full. The vacuum line is sucking. I'm perplexed. Thank in advance for any replies.

It sounds like you have a hydraulic leak in your system. Most likely it's an internal leak of the master cylinder, but check the whole system for potential leaks, i.e. wheel cylinders, brake lines/hoses, etc.