How to Harley Davidson Sportster Bleeding Rear Brake. We recommendation to replace and flush your brake fluid every two years. When just bleeding your brakes to get rid of air, you just add fluid to the master cylinder until you get the air out. You are mixing the new fluid with the old. When you are flushing, You are replacing your brake fluid. You suck the master cylinder dry and then put new fluid in and bleed the brake system until all the old fluid is out, This will work on all Non ABS motorcycles.