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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?
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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 dry but rather just push or draw fresh fluid through to avoid trapped air.
Your thoughts?
Last edited by scottmitchell
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.

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 transparent test set up and witness this for myself.

Maybe both events are occurring: perforation and solutionized air. (Solutionized -- if it is a word, it's a new one.)
Last edited by scottmitchell
Here is a link that explains how to monitor new brake fluid for excessive air by measuring oxygen content.
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 some of 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.

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.
Last edited by scottmitchell
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 tap into my engine's manifold vaccum. 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.
Last edited by scottmitchell
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.
I like the hypothesis of the air absorbing under pressure, then releasing as smaller, more broadly distributed bubbles that can escape.

You get a spongy pedal because a small section of line has a concentrated amount of air fillingf the entire diameter, making it airbound. Push the air into solution and it's now distributed through the whole volume of fluid. If the air then comes back out as smaller bubbles that are much smaller in diameter than the line, they can find their way up to the reservior & leave through the vent.

No data to support it, but it appears to make some sense.
I have to disagree. You have a spongy pedal because the size of the bubbles changes with pressure. PV = PV Raise the pressure by a factor of 10, and the volume falls by a factor of 10. Although any gas dissolves more in any liquid as the pressure rises, I doubt brake fluid dissolves much mostly nitrogen air. It has seemed unlikely all along

Tonight, I remembered that years ago I used a pressurized vessel for airless spraying of acrylic lacquer. We first tried CO2 to pressurize the vessel. It made a big mess of things because the CO2 dissolved into the lacquer expanding the volume and thinning the lacquer. When we switched to nitrogen, it solved that problem. N2 is non polar and very little dissolved in the moderately polar lacquer. Even less is going to dissolve in highly polar brake fluid.

I think we need to look at some of the alternative explanations, not air dissolving in brake fluid.
Let’s apply a little common sense here: “Is unused brake fluid in the container still good a few months later?” It probably is if the cap was sealed airtight. Your brake system is sealed too.
Brake fluid (DOT 3 and 4) is hydroscopic meaning it absorbs moisture. It also absorbs other gasses too. Moisture (water) is the most damaging contaminate to your brake system because it is corrosive and will eventually pit the metal surfaces of the piston walls leading to leakage, so that is one reason brake fluid should be changed. Another reason is it turns brown and begins to crystallize. It crystallizes because it is nearing its maximum limit of absorbing water and other gasses. These crystals cause clogging of the small passages in the master cylinder, most commonly the return vent which is the smallest being about diameter of a pin or smaller. In the worst cases it clogs up everything and you may end up replacing the lines and components. The best practice is to change your brake fluid often enough to avoid this kind of damage. I like to change my brake fluid once a year on all my vehicles.
New brake fluid is dehydrated. As the brake fluid goes through its service life, it begins to absorb contaminates (water and other gasses) turning brown as it reaches its hydration limit. Eventually it reaches a saturation point and the corrosive effects go to work. By then it is long overdue for a change. Pitting and the crystallization are the bad things. The brown color means it needs to be changed and is long overdue.
It is a good thing that brake fluid is hydroscopic and does absorb water and gasses. In a sense the gasses and moisture are neutralized and suspended in the fluid so the harmful effects are delayed. Over time the absorption limit is reached and the excess contaminates are free to do their damage.
The best practice is to use new brake fluid after it has been opened. Once it has been opened it has been exposed to the air and has started absorbing moisture like a dry sponge. I have never seen brake fluid turn brown in the can and have wondered why is this clear fluid considered bad? It’s probably not bad at all, just compromised some from exposure to the air while it was opened or a leaky cap. I would be inclined to use it in a pinch, but would be mindful that the system needs a fluid change soon. Relatively speaking brake fluid is cheap, so start with new brake fluid. Also flush the system if you can – that is the best practice.
Since brake fluid is supposed to be dehydrated from the manufacture, I would imagine that brand new brake fluid varies from one quality control standard to the next. You could possibly be adding new fluid that has more moisture in it than an opened can of fluid in the garage. Both look clear, both are still serviceable. You just do not have a meaningful way to know how much moisture has been absorbed. I have not found any products that are available to measure contamination of brake fluid, so the best practice is to use new fluid. The only way to know is by the shades of the brown color or if it looks cloudy. It’s cheap, so add or start with new fluid.
This discussion was about brake fluid absorbing air and moisture. Gasses enter fluids under pressure and exit when pressure is relived. Unwanted gasses and excess moisture in a brake system are undesirable because excess gasses lead to spongy brakes or clutches and moisture to corrosion. Generally speaking new brake fluid is more ideal than unused brake fluid because it is dehydrated and has the fewest contaminates. Less contaminates means fewer unwanted gasses entering and exiting the fluid while in service. As it gets older it absorbs contaminates like a sponge. Eventually the fluid like the sponge becomes saturated and will hold no more. To avoid saturation change your brake fluid often and the costly repairs.
I hope this helps.
Last edited by scottmitchell
Thanks for the responses. I flushed out the brake system for my GF. I followed a buddies advise who has the same vehicle. I used close to 3 qts for the job and have half a qt remaining. It is tightly sealed, now I need to replace a caliper on my P/U and wanted to use that up. Cost is not the problem, it just irks me to recycle half a qt of brake fluid. OTOH, for $3.99 for a qt of brake fluid I'd rather not screw up a braking system either.

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: 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.
Last edited by scottmitchell
Brake fluids are glycol ethers. A glycols are HC chains with an OH group at either end (and are sometimes called di-alcohols); ethers are HC chains with an oxygen in the middle of the chain. A glycol ether would have both.

Glycols are hygroscopic, normally a downside, but actually a plus as a brake fluid. It prevents the formation of a two-phase liquid where the water phase could freeze. Their other main advantages are: they do not degrade rubber parts (found all through the brake system) like mineral oil and they have an astoundlingly high VI, so viscosity remains fairly stable through the entire operating range.

Is there data to support that the insoluble brown portion is glycol ether & water forming a crystalline matrix? I always figured that was iron oxide & other particulate matter. Did you try filtration before vacuum dehydration?

Oh, there's no way you'll get 29"Hg from your car, or even most lab vacuum pumps. They top out at about 26"Hg. To get higher than that, you'd need a cascade system.
Much of my curiosity about brake fluid started about ten years ago when I replaced the front pads on my 1985 Goldwing. After flushing the system and refilling it, the front brake was spongy. The pump and squirt method was not getting the trapped air out. After looking at the plastic vacuum bleeders on the market and the price, I decided to make a homemade vacuum bleeder using engine vacuum – it really worked out good. I have since posted the idea if anyone cares to build a free or dirt cheap power vacuum bleeder. See this link:

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