need to know the symptoms of air in brake lines on a 2011 C300 mercedes benz 3.0 luxury sedan?

What is your question?

the other day i was driving my car when from anywhere the brake pedal gets really heavy to stop the car, it does not goes down to stop the car fast three hours later it was fine. We have been serve the car for a problem in the front suspension where they supposed to remove front rotors and brakes , i think is air in the lines, and want to know the symptoms of this kind of trouble.