The pilot was landing his tailwheel-equipped airplane on a dry, gravel runway at the conclusion of a local area flight. After landing, the pilot applied the brakes to slow the landing roll, and while depressing both brake pedals, the left brake pedal unexpectedly went to the floor, and the airplane swerved to the right. The airplane subsequently went off the right side of the runway and nosed over in an area of soft, marshy, wet tundra, sustaining substantial damage to the rudder, left wing, and left aileron. A Federal Aviation Administration (FAA) airworthiness inspector's postaccident inspection revealed that a hydraulic brake supply line fitting, near the airplane's fuselage, had broken. The FAA inspector noted that the hydraulic brake line was not secured to the landing gear strut as required, which allowed the line to flex and bend at the fitting during flight. The pilot reported the he recently rebuilt the airplane himself.