Maybe everybody knows exactly how to do this but here is a handy tip for saving time when you want to bleed brakes or change the fluid.

On race bikes you must change the brake fluid once a year. On the old racers DOT3 is specified but is better to use DOT4. In that case it is advisable to clean the whole system first. You can do that even with washing-up liquid from the kitchen. As long as everything is nicely clean and dry it is okay. Fit cups, pistons, etc. back by using brakegrease. Assuming that you have a bike with a working system the quickest way to change the fluid is by using a "Mityvac" tool.

Mityvac brake bleeder