DIAGNOSIS The brake band mechanism is binding due to sawdust, dirt, or a lack of lubrication. The pivot points for the brake lever and the band itself are gummed up. This is a maintenance issue, not a part failure.
FIX Thoroughly clean the entire clutch cover area. Remove the side cover. Manually work the brake lever back and forth while spraying a degreaser like brake cleaner into the pivot points. Flush out all debris. Apply a light machine oil (not grease) to the pivot pin and the areas where the band contacts the guide plate. Operate the lever dozens of times to free it.
IF THAT DIDN'T WORK The brake band may be deformed or the return springs are weak or broken. You must replace the chain brake assembly. This includes the band, springs, and lever mechanism as a kit, part # 537160601. Ensure the band is correctly routed around the clutch drum during reassembly.
HEADS UP Never operate the saw with a malfunctioning chain brake. In a kickback event, it will not stop the chain. Test the brake by running the saw at idle and slapping the front hand guard forward. The chain should stop instantly.