2000 Ford Explorer Intermittent fading brake pedal

This problem has been going on for some 18 months. The brake pedal will intermittently fade to within 1/2" of the floor before the brakes grab. Releasing and reapplying the brakes yields normal brake pedal travel and action. This happens frequently after climbing a moderate hill or spending some time at speed on the freeway. It rarely or never happens when driving around town or with frequent accelerator changes.

I have replaced the master cylinder twice, the ABS module once and the rear speed sensors once with no success.

With my limited knowledge it seems that it might be a vacuum problem but I have been assured that the power boost checks out okay, no leaks.

The last repair, the speed sensors, came from a shop which claimed to have checked with a Ford specialist who said this was not an uncommon problem with the speed sensor. My local Ford Dealer has never heard of the problem.