Brakes failing at random, any ideas?

At random times, while driving, if I press the brake pedal, no reaction from the brakes. Pump the pedal two or three times and the brakes are fine. No brake fluid is missing, no leaks could be found. Bleeded the system twice already. When I keep the brake pedal pressed while the car is not moving, and the brakes are performing as it should, the pedal stays constant. This tells me that there is no brake fluid leaking past the seals in the master cylinder.

During a brake failure, when I press the brake pedal hard, the right front wheel, locks up. No other wheel seems to braking.