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 it 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?

Yes, though I can't state so categorically. Given my background in chemistry I have a pretty good feel for gases dissolved in liquids. As a rule of thumb, when the pressure rises, more gas can be dissolved in a liquid.

I have a hard time believing this is your problem. If you have air in the lines, somebody is fooling you by telling you to temporarily pressurize the system to dissolve the gas. Once pressure is released the gas will come out of solution. This is due to a phenomenon called "equilibrium". This is also analogous to the "bends" that divers get, where excess air is dissolved into a diver's bloodstream under pressure, then released when the pressure is removed.

Kestas, thank you for taking the time to answer.
Let me add some more now that a track of thought is established.
I have read about pressurizing the system as a remedy for spongy brakes. No one has stated why this works. After reading one person’s explanation that it worked for no known reasons, I decided to try it just to see for myself. I did not believe it would work, but it took very little effort to wedge the pedal down and try. The next day the brakes were fine. I do believe something happened and I do not know what. I have only done this once and now I am very curious to find the scientific reason. I would like very much to know for sure.
I can add this. If you are correct and this is similar to the bends, all the air that is released from solution is then dispersed over the entire internal area. If this is true, then a large majority of it will be able to escape via the brake vent in the reservoir. Not all of it will migrate back to the original high spot, but some will. I should repeat this process a few more times and see if there is more improvement. I’ll do this tonight.
If this is true air is compressed into the liquid and then released, what happens to the moisture that was in that bubble of air? I’d venture to say some of it is captured for good in the new fluid. So the new fluid is now compromised and another flush is in order. This time I will not flush it dry but rather just push or draw it through to avoid trapped air.
Your thoughts?

Saturday afternoon I reapplied pressure wedging the pedal down. Today at noon I released the pedal and it was firmer. I doubt there would be anymore to gain. It feels as good as it can get. The brake engages at half inch of pedal travel and is fully engaged at 1-inch. The pedal bottoms at 2-inches.
The best way to describe the results is by performance. After bleeding with vacuum suction the brakes worked well with a long pedal throw -- about 3/4s of full travel.
After applying pressure the first night the pedal throw was about mid-way with stronger stopping power.
After applying pressure the second night the pedal throw is as short as it can be and is very responsive. It’s far better than I am used to.
I really do not have an explanation as to why this worked, but the results were surprising. I can only surmise the air is absorbed into the fluid under pressure. When the pressure is removed the gas leaves the solution over a broader area and much of it is able to escape out the system’s vent. Some of it returns to the original location. Maybe some it stays absorb in the solution as each solution has a saturation limit. New brake fluid may be dryer and is thus able to absorb more.
What I write here is only my best guess as to what may have happened – I really don’t know.

Kestas' explanation makes sense.

In my own playing around I have put some water in a syringe and then capped it with my thumb while pulling the plunger.

You can then watch the air come out and back into solution.

A very easy and practical demo of the concept.

As for the tie down method, intriguing.

Could the time allowed be enough for even distribution of the solutionized (is that a word?) air?

Something is happening, but what?

I have a discussion going on over at MythBusters. http://community.discovery.com...919888&m=72319093301

There I have explained the facts and some responses that I think are misunderstood.

The systems high areas are rubber lines that rise above both master and slaves. If the air is trapped in rubber lines, then it may be escaping by perforating the rubber line, but the fluid can not. I have examined the system once again for leaks, but here are none and the fluid level is the same.

Your explanation sure does help. If had the resources, I'd too like to make a clear test set up and witness this for myself.

Maybe both events are occurring: perforation and solutionized air. (Solutionized -- it is a word now and I don't know either.)

Here is a link that explains how to monitor new brake fluid for excessive air by measuring oxygen content. http://www.freepatentsonline.com/5361624.html
It mentions new brake fluid is dehydrated, thus readily absorbs air. When new brake fluid is injected into new vehicles bubbles are inevitable. The dry fluid absorbs these bubbles, so it is not necessary to bleed the system afterwards on the assembly line.
Perhaps this has something to do with why the overnight tie-down method works. The applied pressure which can be as much as 500-psi or more helps the fluid absorb air into solution. As the pressure is released or decreases over night from leaked-down the now hydrated fluid retains the air. The dry brake fluid acts like a sponge until saturated.
• This link says the system for testing brake fluid must have an upper limit of 1000-psi for testing. http://www.nhtsa.dot.gov/stati...0Files/TP-116-04.pdf

Maybe as the brake fluid ages its saturation is greatly reduced, thus reducing the effectiveness of this method.
I still don’t know and will continue searching for the answer.

Solutionized is usually used when discussing molten metal.

Interesting. I am never one to deny what somebody reports. I once managed a small factory. I always hated it when the foreman came in my office reporting something happening out on the floor that was totally impossible. I knew if i went out there, it would be just as he said and I would have to figure it out.

Air will leak out of a system where brake fluid never would. My guess is that under pressure, the air slowly leaks out. I have seen spongy pedal slowly get better on its own. I always though it was the air slowly working its way to the top.

You might find this interesting: After flushing the fluid on my 1997 F150, I wanted to be sure all the air was completely removed so I pulled a vacuum on the reservoir. I used an orange transparent pill bottle to cap the top of the reservoir. The round opening of the reservoir and the pill bottle matched up for an air tight seal. I drilled a ¼” hole in the bottom of the pill bottle, inserted ¼” vinyl tubing and connected the other end to an engine vacuum line pulling 18 inHg.
When I placed the pill bottle on the reservoir there was an instantaneous fog in side, but no bubbles coming up. It looked like vapors, but it also could have been condensation on the inside or both. It remained dense for about twenty seconds then began to dissipate until it was gone. I think it was moisture vapors out gassing from the brake fluid, same as pulling a vacuum on an AC system to remove moisture. Whatever it was, I don’t think it should have been there and it makes me wonder just how dry was the new brake fluid. This proves nothing except that I saw vapors that I could not identify, but it was certainly some type of out gassing.
From now on when I service my hydraulic systems, I am going to flush the lines dry and then pull a vacuum on the empty system to remove moisture. If it is done on the assembly line, it is probably a good idea. After filling it with new fluid, I’ll pull a second vacuum to boil off any moisture that may be in the new brake fluid. New fluid may not be as dry as it should. It all depends on the quality control of its manufacture and we all know how things slip past.
To pull a vacuum I have use simple plastic food container lids with a ¼” hole drilled and a ¼” vinyl line inserted. The vinyl line fits snugly in the ¼” hole and does not need anything to seal it. Sometimes the hose does creep inward when the vacuum reaches maximum. These simple plastic lids work pretty good sealing off the smaller openings up to about 1 ½ inches in diameter. This is a very simple and effective method for pulling a vacuum. I used the pill bottle hoping to see bubbles and was lucky it matched up and sealed. Just for fun, I pulled a vacuum on a plastic 64 oz Hellmann's Mayonnaise jar. It took about 10-seconds to flatten it.

Does the pressure have anything to do with early wear on brake hoses? mine gave up on me so i bought myself an AC Delco Brake Hose as replacement for my oem brake hose.

This message has been edited. Last edited by: archebald23,

The harder you press on the pedal, the more pressure. the more pressure, the more you stress the hose and other components. I think the deterioration of rubber with time is the biggest factor, but hard brake usage could lead to earlier failure. Dissolved air contains oxygen, another enemy of rubber.

Brake hoses are one more thing that eventually need replaced.