A brake pedal can be hard to push because of several factors, including a brake pedal getting caught on something, failure of the power brake booster or a sticking shoe, which causes the brake fluid to boil. Additionally, a brake can be hard to push if water builds up in the brake line or as a result of a restriction in the brake line.
Restriction in the brake line is caused by either a bent brake line or foreign material in the brake fluid. A common cause of a hard brake pedal is a sticking brake pad or shoe. When a braking pad sticks, it causes the brake fluid to boil. When the brake fluid boils, it expands and causes the hard brake pedal that one feels.
It is very hard to track down the causes of a hard brake pedal. The easier method is feeling the brake lines of the vehicle after a test drive. However, brake lines are always hot after a drive, which makes diagnosis tricky. Failure of the power brake booster is a serious cause of hard brake pedals. It is advisable to get help from a professional vehicle mechanic in the event that a brake pedal becomes hard to push.