Final answer:
The most common cause of parking brake failure is driving with the parking brake on, which leads to excessive wear and overheating of the brake system.
Step-by-step explanation:
The most common cause of parking brake failure is driving with the parking brake on. When the parking brake is engaged and the vehicle is driven, it can lead to the brake pads or shoes wearing out prematurely. Additionally, this can cause overheating, which may result in the brake system losing its effectiveness. The other options listed, such as parking facing downhill or uphill, do not directly cause parking brake failure but are rather considerations for using a parking brake correctly to ensure a vehicle remains stationary. Pulling the parking brake lever too hard over time can potentially lead to mechanical issues, but it is not as common as the wear and tear caused by driving with the brake engaged.