BE55 pilot on a training flight reported main landing gear brake locked up on takeoff roll; which caused a runway excursion during the rejected takeoff maneuver.

Narrative

We were lined up for takeoff on runway XX. Our taxi and run up were all smooth with no issues. We put the throttle to 2000 to initially pick up speed before going full power. At this time; everything was normal. Once going full power; we started to veer off to the right. I was hitting left rudder to try and line back up; but it did not work. We originally thought we had popped a tire and went power idle. Once idle; the plane continued to veer to the right as we came to a stop several feet off of the runway. Our next thought was that the brake had locked up; since there was a distinct dark skid mark from the right tire and not the left. Upon initial inspection of the plane; no tires were popped. Once maintenance personnel were able to tug the plane from where we had stopped; the brakes on the right seemed to lock up again. The mechanic went to check them; and noticed that one of the brake pads had become loose and had wedged itself between the brake caliper and the wheel. He then removed them and continued to bring the plane back to maintenance. We believe it was this mechanical issue that caused the plane to veer so sharply to the right.