A bad brake booster is one of the most common causes of a hard brake pedal, a lack of vacuum pressure in the brake booster could also cause a hard brake pedal.
Failing a vacuum pump or power steering pump, damaged vacuum hose or seized caliper could also be the cause.
Sludge in the brake line, brake fluid absorbs water, over time this will build up sludge in the brake system. Brake fluid should be clear.
If your vehicle can't stop or slow down effectively, stop by your nearest auto repair shop for brake inspection and repair.