Something similar happened to me in an old chevy I had... was doin about 75-80 down the highway and when I went to exit, I came up on a 25 mph, u-turn off ramp... I applied the brakes and foot went completely to the floor... I repeated the process a few times and finally built enough pressure to slow for the turn (basically pumped and bled the brakes)... the next morning, I had brake fluid flooding the well of my right front wheel. :\.. busted brake line... The trip home (about 105 miles) was driven at no more than 45 mph, with emergency flashers all the way... had to use my gears to slow and stop the car in the city limits... was not very fun.
If you don't have any cracks in the brake line, I'd have to wonder about your master cylinder.. perhaps it's not pushing the fluid like it should..