Yes, you need to flush your brake fluid at least every 2-3 years. Flushing brake fluid every year may be overkill, but will certainly keep your brakes working for a long, long time.
I usually recommend that you flush your brake fluid at least with every brake job (front, rear, or both) That usually works out to about every 2-3 years for me, but varies by how much YOU drive or the weather conditions where you drive. If you can get 4+ years on a brake job, then definitely do a fluid flush at least every 2 or 3 years.
And yes, it will improve your braking power if you had not changed your fluid in a long time, If you change the fluid regularly over the life of the vehicle, the fluid will be not deterioration as much and you may not notice any difference...which is a good thing since shows that your Preventive Maintenance works. :grin:
...Rich