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Does hydraulic fluid absorb moisture like brake fluid? #5261734 11/08/19 11:56 AM
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atikovi Online Content OP
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Just changed the fluid in my '95 Mercedes convertible probably for the first time, and it was clean with no sign of corrosion on the pump metal parts. The external metal has surface corrosion. Did have some sludge on the bottom of the reservoir so I'm glad I changed it. I'd guess this fluid doesn't absorb moisture. Another use for the MityVac.

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Re: Does hydraulic fluid absorb moisture like brake fluid? [Re: atikovi] #5261740 11/08/19 12:07 PM
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UrS4 Offline
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No.
Most hydraulic fluids are designed to NOT mix with water.

Last edited by UrS4; 11/08/19 12:08 PM.

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Re: Does hydraulic fluid absorb moisture like brake fluid? [Re: atikovi] #5261752 11/08/19 12:20 PM
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So why don't they use them in brake systems? Seems like it would last forever.

Re: Does hydraulic fluid absorb moisture like brake fluid? [Re: atikovi] #5261757 11/08/19 12:23 PM
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Anduril Online Content
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Most hydraulic systems don't reach the very high temperatures brake systems do. I'm guessing they had to change the formulation to cope with that, and put up with the water absorption as a side effect.

The convertible pump on my Mustang takes regular ATF and states it never has to be changed. As it's a closed system under absolutely no stress, I'm inclined to believe that.

Last edited by Anduril; 11/08/19 12:24 PM.

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Re: Does hydraulic fluid absorb moisture like brake fluid? [Re: Anduril] #5261760 11/08/19 12:28 PM
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kschachn Online Content
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I don't understand the temperature thing, regardless of the temperature it still has to be exposed to some moisture for it to absorb water, right? I thought the problem with hot brake fluid is that it creates vapor bubbles which degrade the performance.


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Re: Does hydraulic fluid absorb moisture like brake fluid? [Re: atikovi] #5261761 11/08/19 12:28 PM
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Rolls-Royce/Bentley did use HSMO for their braking and suspension hydraulics. Shimano uses mineral oil for their bicycle hydraulic brakes since it's safer than glycol ether based fluids for the cyclist and environment.

I'm guessing since DOT fluid is a regulated standard and cheap as well is why mineral oil never took off for automotive brakes.

Re: Does hydraulic fluid absorb moisture like brake fluid? [Re: atikovi] #5261763 11/08/19 12:30 PM
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Brake fluid is basically polyethylene oxide, a polyether. It does not interact with rubber which makes possible flexible rubber brake lines and rubber caliper/piston components in the system. I am sure the same could be done with newer elastomers today and with hydraulic fluids but there is the COST factor. The system works, why spend more??


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Re: Does hydraulic fluid absorb moisture like brake fluid? [Re: atikovi] #5261764 11/08/19 12:32 PM
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Anduril Online Content
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There is a reason, from our good friends over at the Ask Science subreddit:

Quote
"The hygroscopic nature of DOT brake fluid is actually an advantage in a major way.

So where does this water come from?

Glycol based fluids are hygroscopic which means they absorb water/moisture from the environment at normal atmospheric pressures at a rate of 2-3% per year. This process is exasperated in more humid conditions and climates.

This water content finds it's way into the brake fluid via microscopic pores in brake hoses, seals, joints and seams. As we've learnt, water mixed with DOT fluid has an adverse effect on the brake fluid by reducing it's boiling temperature and therefore reducing it's performance.

Here is how.

As water enters the system, instead of pooling in low spots (such as the calliper), due to it's weight in comparison with brake fluid, it is dispersed throughout the whole of the brake fluid. This helps to keep the boiling point of the entire brake fluid high rather than having pools of water in the system which will boil much sooner than the rest of the brake fluid.

It also prevents localized corrosion of internal parts which can be caused by water pooling in the brake system.

In some cases, Mineral oil is used in braking systems.

Unlike DOT fluid, Mineral Oil is hydrophobic and does not absorb moisture from the environment. This means that there are no wet or dry boiling temperatures to worry about, the boiling point stays constant and never drops. That's the good news.

The bad news is that any water that does enter the brake system, via seals or microscopic pores in the lines etc., will effectively reduce the boiling point of the whole brake system to that of water - just 100°C. This is because as the fluid repels any water ingress, it causes it to pool at low points within the brake system, usually the caliper, since water is heavier than brake fluid it will settle at the lowest point. This is worrying because the fluid in the caliper is more susceptible to high temperatures as it's at the business end of the brake, where the friction is created."

Last edited by Anduril; 11/08/19 12:33 PM.

2015 Nissan Versa Note 1.6/5MT
2012 Toyota Tacoma 2.7/4AT
2007 Toyota Camry 2.4/5AT
2005 Nissan XTerra 4x4 4.0/6MT
2002 Mustang GT Convertible 4.6/5MT
2009 Kawasaki Vulcan 900
2013 Hyosung ST7
Re: Does hydraulic fluid absorb moisture like brake fluid? [Re: Anduril] #5261785 11/08/19 12:49 PM
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Great, thanks for the cut-and-paste. That does make some sense, at least to why it is hygroscopic.


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Re: Does hydraulic fluid absorb moisture like brake fluid? [Re: Anduril] #5261801 11/08/19 01:10 PM
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Quote
This process is exasperated

I get exasperated with malapropisms.


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Re: Does hydraulic fluid absorb moisture like brake fluid? [Re: atikovi] #5261862 11/08/19 02:51 PM
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Change your thinking...not your oil!
Turn your mind....not your rotors!
Re: Does hydraulic fluid absorb moisture like brake fluid? [Re: pitzel] #5261877 11/08/19 03:10 PM
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atikovi Online Content OP
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Originally Posted by pitzel


Yes good, if a bit dated. No mention of LV brake fluids.

Re: Does hydraulic fluid absorb moisture like brake fluid? [Re: nthach] #5262016 11/08/19 06:32 PM
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Originally Posted by nthach
Rolls-Royce/Bentley did use HSMO for their braking and suspension hydraulics.


Yes and if you bought it the sphere came in a Citroen box, yessiree both RR & B used Citroen suspensions (don't know about brakes though)

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