IT'S A WASTE OF TIME AND MONEY! Don't do it! Brake fluid is in a sealed system, so if you never open the cap on the master cylinder (and since it's clear, you can check the level without doing so), it will never get dirty. If you don't have a problem with the brake system, you should never need to change it.... unless there is a leak or you cook the fluid by doing 20 hard stops from 100 mph in a row.