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See also An Exercise in Bleeding the Brake System
The test subject is a brake lever and master cylinder from a 95 Kawasaki but the process is the same for a Sportster.

Before replacing fluid or bleeding the air from your M/C, make sure the master cylinder has new and clean brake fluid in it.
Remove (soak up) any old fluid with a rag or paper towel and wipe down the inside of the M/C.

Then add the proper fluid to the fill line.
If in doubt about what type fluid to use, refer to your FSM and it's also stamped on the cover.
See more information here about Brake Fluid in the REF section of the Sportsterpedia.

You can see the difference in the color of the old and new brake fluid in the reservoir.
You don't want to send more old brake fluid in the system while you are working.