You should know that your brakes are fading when:

A You must push harder on the brake pedal to control your speed on a downgrade.

explanation

Increasing application pressure to hold the same speed means the brakes are fading. You should slow down and use a lower gear. The need for increased pressure can also be caused by brakes out of adjustment, air leaks, or mechanical problems.
[CDL Handbook: The Parts of an Air Brake System]