Narrative:

During a standard departure from ZZZ; my aircraft veered to the left off of runway xxl into the gravel apron between the runway and taxiway a coming to a stop on taxiway C.preflight check was normal; refueling went normal; run-up was normal; contacted ground from the fuel pump and was given taxi instructions to [runway] xyl via east; and alpha. Taxi went normal. Contacted tower that I was holding short of [runway] xyl at a. I was then given clearance to take off on runway xyl with a left turn out and departure to the southwest.my aircraft is equipped with 'finger brakes' located on the center console. I entered the runway and used differential braking and rudder; turned left to line up with the runway centerline. Once lined up but still rolling; I gradually applied full power and some right rudder. As the plane quickly moved forward; it started to veer to the left. I applied more right rudder until I had all right rudder and still only managed to decrease left turn.the plane left the runway between [taxiways B and C]; but once on the gravel came more to the right and I was able to avoid any runway lights or signs. Its momentum carried me onto the taxiway C intersection in between runway xyl and taxiway a where I stopped. I contacted the tower and advised them that I had aborted the takeoff and requested taxi back to parking. During the taxi back; the aircraft had a distinct tendency to turn left. When I put my hand on the brake levers; I felt a misalignment between the levers. Further inspections showed that the finger knob on the left lever had jammed behind the finger knob on the right lever. This caused the left brake to drag while taxiing enough to cause the veering tendency. Once they were unjammed; normal steering resumed. I taxied to parking and performed a detailed inspection of the aircraft. With the exception of some dings to the paint on the rear horizontal tail plane; no other damage was noted. With this issue in mind; I continued my flight uneventfully with several landings and takeoffs with no problems.after returning; I secured the aircraft and began to engineer replacement parts to prevent this issue again. I manufactured two replacement knobs that will not be able to jam and repeat this problem. The following week; I performed repairs to the paint dings. I also replaced both finger brake knobs and exercised the finger brakes several times and am confident this problem will not be repeated.

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Original NASA ASRS Text

Title: Experimental aircraft pilot reported a brake malfunction and runway excursion during takeoff roll.

Narrative: During a standard departure from ZZZ; my aircraft veered to the left off of Runway XXL into the gravel apron between the runway and Taxiway A coming to a stop on Taxiway C.Preflight check was normal; refueling went normal; run-up was normal; contacted Ground from the fuel pump and was given taxi instructions to [Runway] XYL via E; and Alpha. Taxi went normal. Contacted Tower that I was holding short of [Runway] XYL at A. I was then given clearance to take off on Runway XYL with a left turn out and departure to the southwest.My aircraft is equipped with 'Finger Brakes' located on the center console. I entered the runway and used differential braking and rudder; turned left to line up with the runway centerline. Once lined up but still rolling; I gradually applied full power and some right rudder. As the plane quickly moved forward; it started to veer to the left. I applied more right rudder until I had all right rudder and still only managed to decrease left turn.The plane left the runway between [taxiways B and C]; but once on the gravel came more to the right and I was able to avoid any runway lights or signs. Its momentum carried me onto the Taxiway C intersection in between Runway XYL and Taxiway A where I stopped. I contacted the Tower and advised them that I had aborted the takeoff and requested taxi back to parking. During the taxi back; the aircraft had a distinct tendency to turn left. When I put my hand on the brake levers; I felt a misalignment between the levers. Further inspections showed that the finger knob on the left lever had jammed behind the finger knob on the right lever. This caused the left brake to drag while taxiing enough to cause the veering tendency. Once they were unjammed; normal steering resumed. I taxied to parking and performed a detailed inspection of the aircraft. With the exception of some dings to the paint on the rear horizontal tail plane; no other damage was noted. With this issue in mind; I continued my flight uneventfully with several landings and takeoffs with no problems.After returning; I secured the aircraft and began to engineer replacement parts to prevent this issue again. I manufactured two replacement knobs that will not be able to jam and repeat this problem. The following week; I performed repairs to the paint dings. I also replaced both finger brake knobs and exercised the finger brakes several times and am confident this problem will not be repeated.

Data retrieved from NASA's ASRS site and automatically converted to unabbreviated mixed upper/lowercase text. This report is for informational purposes with no guarantee of accuracy. See NASA's ASRS site for official report.