Brake Pedal feels hard

[M5Gonz]

Several months ago I started feeling my M5's brakes pedal very hard. The car would stop, but I had to apply more pressure than normal (in my mind) to make the car to slow down or a dead stop. I decided to take the car to the shop for a brake fluid flush. After that, the pedal felt normal. It lasted like that for around 3 weeks, then started acting in the same way. This time I brought the car to a mazda dealer and they told me that the brake system is fine and normal. ABS/brake/check engine warning lights are not flashing, pads/rotors are ok and no leaks. Anybody had/having this problem? Any advise or idea? Thanks

By the way, my M5 is a 09 with around 58,000 miles and a manual transmission.

Thanks