As explained to me when I had my 924 in the 1980's brake fluid is HYDROSCOPIC which means it absorbs moisture just by being exposed to air. This is one reason a new can of fluid is sealed and is clear when you open it. After time it turns darker. Both air and water compresses easier than brake fluid which might contribute to a soft pedal.
When I used to flush my system with, if you could find 2 others to help you, by the old pump the brake pedal all the way to the floor, half way up and all the way to the floor etc etc, I would first drain the master cylinder by using a turkey baster with a long tube on it and suck out as much fluid as I could. Then I would fill it with brand new fluid. One person by the master cylinder to pour new fluid into the master cylinder as the level went down, another pumping the brake pedal, and me under the car. Going to the wheel the farthest point from the master cylinder I would put a clear plastic tube over the brake bleeding nut and insert the tube into a glass jar with about 1 inch of new fluid into it. I would open the nut. as the person pumped the pedal, I would see the dark colored fluid go into the tube and into the jar. When the pedal was all the way to the floor I would close the nut, the person would allow the pedal to come up half way up, I would open the nut with the tube on it and the pedal would go to the floor again, until there was only clear fluid in the tube I would close off the nut and go to the next wheel rear wheel. By keeping the end of the tube submerged under the 1 inch of fluid at the beginning the tube would not draw air up the tube.
It was before the Motive power bleeders existed but it worked.
The dark fluid in sealed brake lines was because the fluid had absorbed moisture over time and from the heat of the brakes. By flushing my system once a year I always had a hard brake pedal, no rust near the brake cylinders and pistons with great stopping brakes all the time. Brake fluid is cheap compared to the other alternative. Why take a chance with an opened can of fluid. No matter how tight you sealed the cap or what the temperature is in the garage it was opened and exposed to air and moisture.
William Moss
Tigard Oregon
