Brake fluid musings

[Craig380]

I last changed the brake fluid on my 2017 Gen 3 (with ABS) in December 2021. Yesterday I thought I'd check the condition of the fluid out of curiosity. The moisture level was still below 1%, which is still well on the safe side. The fluid is just Comma DOT4 (£6 a litre from the local motor factors), nothing special.

I do around 11 to 12K miles per year on the bike and while it is garaged, the garage is not heated and is a bit draughty. It shows that in the UK climate, even basic brake fluid can last well beyond the usual recommendation to change / flush every 2 years.

[Bibio]

i've services bikes where the fluid has been black and the brakes still worked perfect. i personally think its the manufacturers covering their behind or its just to sell more fluid.

[Craig380]

I think in some climates with high humidity / high temperatures, brake fluid will go off quickly ... but not in the UK!

[glang]

I do like the flexible rubber seal used in motorcycle master cylinders and I think this helps keep the fluid dry compared with cars which in general just have an vent open to atmosphere to allow for changes in fluid level...

[admin]

I changed the fluid in my VFR every two years. I could see the fluid going darker as it aged. I could definitely feel the difference after changing. Also ABS pumps don't like moisture and can corrode, simpler to change fluid than a frozen ABS unit.

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[redtrummy]

My mechanic, if I suggested a brake fluid change on the car would ask 'why'

[glang]

My SV was completely underwater for more than 5 days so I changed the brake fluid but probably didnt need to...

[SV650rules]

I do like the flexible rubber seal used in motorcycle master cylinders and I think this helps keep the fluid dry compared with cars which in general just have an vent open to atmosphere to allow for changes in fluid level...




Automotive brake fluid reservoirs have a vent in the actual lid, but below the lid is a flexible membrane that has a collar ( sometimes pressed down by the lid to seal better) that is a tight fit in the neck of the reservoir and the membrane will allow level in reservoir to rise or fall while sealing the fluid from the atmosphere... The pinhole vent in the actual lid is just to allow the air above the flexible membrane to equalise and not build up pressure or vacuum as the membrane does its job. I have had a MityVac for may years and bleed the brakes on our cars and bikes about every 3 years. The MityVac makes it super easy to vacuum bleed the brakes - vacuum bleeding means that any air bubbles in the system will grow a lot larger under vacuum and be easier to suck out, and as long as you keep the vacuum on as you retighten the bleed nipple any remaining air bubbles will be massively reduced in size when subject to atmospheric pressure.


https://www.amazon.co.uk/Mityvac-MV8000-Test-kit/dp/B00265M9SS/

[SV650rules]

I changed the fluid in my VFR every two years. I could see the fluid going darker as it aged. I could definitely feel the difference after changing. Also ABS pumps don't like moisture and can corrode, simpler to change fluid than a frozen ABS unit.

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Agreed, flushing out old fluid is just good maintenance practice, and can prevent much more expensive problems happening. I have used a MityVac for many years now and it makes brake flushing super easy and clean, and a proper 'one man' job, even on cars.

[glang]

Automotive brake fluid reservoirs have a vent in the actual lid, but below the lid is a flexible membrane that has a collar ( sometimes pressed down by the lid to seal better) that is a tight fit in the neck of the reservoir and the membrane will allow level in reservoir to rise or fall while sealing the fluid from the atmosphere... The pinhole vent in the actual lid is just to allow the air above the flexible membrane to equalise and not build up pressure or vacuum as the membrane does its job.

Hmmm Ive never seen one of those in a car only on bikes and I assumed it was because theyre more exposed to the elements but maybe it depends on the manufacturer...

[SV650rules]

Sometimes the rubber diaphragm stays inside the cap when you remove it, on other reservoirs it stays in the neck of the reservoir and you have to pull it out to add fluid, there will often be a plastic filter in the bottom of the reservoir to stop any bits that may be in brake fluid ( who re-uses brake fluid ? ) or maybe a bit of rubber breaks off the cap or diaphragm and filter will catch it. John T is correct that fluid does darken in the system, this may not be from water, just aging / contamination of the fluid... You can clearly tell old fluid by it being darker, and sometimes by the smell.

[glang]

You are indeed correct Sir! Ive just checked my Sandero and as you say theres a rubber diaphragm up inside the cap - does make sense...