An RV7A's right brake failed as it was taxied for takeoff because brake wear allowed the brake piston to become dislodged from the caliper draining all fluid to the brake.

Narrative

I had been cleared by Ground Control to taxi to Runway 26R via A taxiway from transient parking and fueling area. Taxi began normally. Nearing the end of taxiway; a left braking control was used to begin the left turn to hold short line at Runway 26R. As out brake control was put in to straighten the aircraft; the brake pedal went to the floor with no braking action. Because left turn was already initiated; the aircraft continued in a left direction. The aircraft slowed somewhat and ran off taxiway with the left wheel pant/fairing hitting a taxiway light. The aircraft stopped as the wheel(s) went into the dirt. Investigation showed that the right side wheel brake piston had dislodged from caliper. A combination of worn rotors and worn brake pads allowed the piston to move beyond the sealing 'O' ring - all brake fluid in the right side was lost. Brake pads were last replaced almost a year ago and rotors met reuse guidelines. Approximately 80 hours had been put on new brake pads. To prevent recurrence advise that inspections be carried out more often. New rotors; pads and piston seal were installed as corrective action.