On September 08, 2013, about 1030 eastern daylight time, a Piper PA-31P airplane, N57JK, was substantially damaged following a runway excursion during landing at Doylestown, Pennsylvania (DYL). The commercial pilot was not injured. The flight was conducted under the provisions of 14 Code of Federal Regulations Part 91 as a personal flight. Visual meteorological conditions prevailed and no flight plan was filed for the flight. The pilot reported that, following a normal landing on runway 5, the left brake pedal went to the floor and there was no braking action on the left side. He tried pumping the brakes to regain left braking action, but was unsuccessful. The airplane departed the right side of the runway, into the grass. The landing gear collapsed and the airplane came to a stop. An inspector with the Federal Aviation Administration responded to the accident site and inspected the airplane. He confirmed substantial damage to the left wing spar. The left brake was not operational when he inspected the airplane. Further examination of the wheel brake system revealed a leak at the shaft for the parking brake valve. He reported that this condition allowed air to enter the brake system in between the master cylinder and wheel cylinder when the aircraft was pressurized.