Traction Control issue

My son has a 2009 Prius that we recently replaced the front right side wheel bearing on and replaced the brakes on both the front right and left sides. Now he is having an issue when he is breaking. Around 4-6 mph while braking the front right will shudder briefly as if the Traction control or ABS is activating and sometimes the Traction control light will come on and sometimes it does not. How do I test the sensor to see if it is causing the problem or is there something else I should be investigating?