Brake problems: My brakes work fine.

Brake problems:
My brakes work fine driving in the streets, but in the freeway after a certain number of miles, the break pedal goes to the floor when I apply it, and I have to pump it a few times. The master cylinder is brand new, there is no leakage in the brake cylinders or the master, the brake fluid level is always the same. I've bleed the four brakes more than three times already, and the problem won't go. I suggested that maybe it's the booster, but they tell me that if it was the booster, the brake pedal would be hard to press.
Please help, Sal.