Brake fluid is hygroscopic, but will it also absorbs air bubbles under pressure? Can brake fluid absorb air and if it can, how much? Is there a volume ratio? I have read on a few motorcycle forums that if you change the brake fluid and have spongy brakes, you can tie down the lever or pedal over night and this will remove the air. If it does, where does it go? Is it absorbed? Does it migrate through the fluid and out the vent or through the rubber lines? I tried this a few nights ago and it worked. Now I would like to know why.
Here are the facts: 1985 Honda Goldwing. Fluid: DOT-4. The rear brake pedal powers the rear caliper and one front caliper. The second front caliper is operated by the hand lever and is not part of this discussion. The reservoir and master cylinder sit low just above the swing arm. The rear and front brake hoses travel up and above the master cylinder. The front brake line travels up and over the front folks. This area is known to be a difficult area to remove air with the pump and bleed method. I used an 18 Hg vacuum bleeder to pull the new fluid through. It moved along pretty quick, but still left some air behind. The brake worked well, but with a long pedal throw and could almost be pushed to its full limit. It felt elastic. I am assuming it still had a bubble or two trapped in the high lines – especially the one going over the front folk. The rear wheel still could be locked at about 2/3s travel and the front caliper gave good results.
I decided to try the overnight tie-down method -- 24-hours. I used a board to wedge the pedal at maximum. The next day the pedal was firm and the travel was normal and stops at mid-way. It’s now correct.
I do not believe the air was pushed out the reservoir vents, because it would have to have traveled down ward.
I know little more than brake fluid is hygroscopic. Does this mean other gases too?