a previous post reminded me to share this with the group:
I had to replace brake fluid last year, and decided that it would be easiest when I had a one-way valve to avoid sucking air into the system when releasing the brake pedal.

Simple solution: take a one way valve like from the fabulous windshield washer system, a 15cm tube, and a small container for catching the fluid. Put the one way valve on the tube, tube on the brake valve and open the valve. Slowly push the pedal, and first time slowly release the pedal. The fluid will be pushed out of the one way valve which prevents air sucking back in. Keep the end of the valve in the fluid. From there on its just making sure to fill up the reservoir in time. Be aware that the rear brakes have a bit more fluid in the lines to replace too.

Simple solution, costs nothing if you have a leftover one way valve at hand.
Works great if you do the job alone.

Theo Jenniskens
1988 928s4 cherry red