Here is another interesting experiment I did with old, brown, brake fluid. I placed it in a sealed jar, applied engine vacuum to it for about fifteen minutes to see if I could boil out the moisture and make it clear again. Under vacuum, water boils near room temperature. See the chart on this page: http://www.aircondition.com/tech/questions/38/. The water boils turning to steam or a gas and thus is able to be drawn out of the fluid. This is the same principle used to pull a vacuum on an air conditioning system to remove moisture. The engine vacuum was about 20 HG and the air temperature was over 90 degrees. Idea would have been closer to 29 HG.
The brake fluid looked clear in the reservoir before removal, but looked fairly brown after it was removed from service. The results: the brown color remained and there was no change at all. From this, I would conclude old brake fluid is contaminated with other impurities and cannot be restored to new condition. It was not my intention to reuse old brake fluid, but just to see if removing moisture would restore the clear color out of curiosity.
My conclusions are:
• New brake fluid is dehydrated. Think of it as a dry sponge.
• Dehydrated or dry brake fluid will absorb air and most importantly moisture. This is similar to the dry sponge.
• Automobile manufactures store brake fluid under a vacuum to keep it dehydrated. The assembly line pulls a vacuum on the empty brake system, and then add dehydrated brake fluid under vacuum so that it is able to absorb the inevitable air bubbles in the system thus avoiding manually bleeding the brake system – this saves on production cost. Only a small number have to be pulled aside to be manually serviced. The articles (links) explaining this are in the above post. They were my biggest clues, along with the other ideas above, and my owning experimenting.
• The properties of dry brake fluid allow a percentage of moisture to be permanently absorbed and maybe some gasses too. Gasses enter and exit solutions (brake fluid) as pressure is applied and relieved. Over time the brake fluid becomes statured and contaminated permanently. This is similar to the sponge holding its limit.
• Small amounts of trapped air can be absorb into brake fluid under pressure and released over the broader area of the entire system allowing some of this trapped to escape the system’s vent. By this nature, brake systems are self bleeding to some extent.
• My original question was: why does the over-night tied-down method remove trapped air. The above explanations seem to answer that question. On my 1985 Goldwing the master cylinder feeds the rear brake and one front caliper. The master cylinder sits lower than the brakes lines and by design is prone to trap air even with vacuum suction. New brake fluid that is supposedly dry is added to the empty system under vacuum suction. A few air bubbles are trapped in the high lines thus the brakes are spongy. The over-night tie-down method applies much pressure to the system over night. I believe the air is absorbed into the (new) dry brake fluid. When the pedal pressure is removed the next day, the absorbed air is released over the entire internal area allowing some of this air to escape out the system’s vent in the master cylinder. One or two more application effectively removes most all the unwanted air so the system's performance is excellent. My own test proved this to happen. Under normal use the system is able to purge trapped air. It has been over a year and the brake system is still good and has not leaked any fluid.
• Under normal usage the brake fluid will eventually reach its saturation limit because of other factors not discussed here and thus should be periodically changed to get the maximum effect of dry brake fluid. The solution is to frequently change your brake fluid.