Brake Fluid 101

[budd]

I've just spent the last 1/2 writing a post re a brake fluid question on RSV'R.net, as the info is just as relevant to the Honda as the Aprilia I thought I’d share.


Question/comment originally posted by kiwi_rsvr kiwi_rsvr wrote:

5.1 is a sort of upgrade to 4, you can mix the two , just never ever use 5


It’s not quite that straightforward, firstly you’re absolutely right NEVER mix DOT 5 with any other type of fluid it’s silicone based and doesn’t play well with others, the others are DOT3, 4 and 5.1, for the purpose of this post it’s probably wise to ignore DOT 5 which is very rarely used in mainstream automotive applications anyway.

DOT 4-5.1 are by far the most popular fluids and are found in the majority of vehicles, DOT 3,4 and 5.1 are all poly glycol based and therefore can be mixed freely, the main difference between them is boiling point, the DOT spec states DOT 3 205 deg DOT 4 230 deg C DOT 5.1 260 deg C (dry)*, although it’s worth noting that these are minimum values and manufacturers can and do produces fluids that comfortably exceed these values, Motul RBF 660 DOT 4 is a great example of this, rated at 325 deg C (dry) it not only surpasses the DOT 4 min spec value by nearly 100 deg C, but also that of DOT 5.1 by 65 deg C !!

I said above boiling point was the main difference between the different grades it is however not the only difference the other difference is viscosity, brake fluid viscosity is measured in centisokes (CST) DOT 4 value is 1800CST whereas DOT 5.1 is 900CST so DOT 5.1 is substantially thinner then DOT 4, at a given temperature, flow / stability at very low temps is vital in order to preserve the performance of ABS, TCS, ESP etc
So thinner DOT 5.1 is ideal for brakes systems which feature ABS, Traction / Stability control systems where a thinner fluid improves flow through the complex maze of valves, actuators, regulators etc that make up a modern brake system esspecially important at very low temps. The benefits of a thinner fluid in the Aprilia set up are less clear cut, being a relatively simple non ABS set up the fluid doesn’t need to be so thin as there isn’t a morass of pipe work and valves to negotiate so my belief is that a V high boiling point DOT 4 would be the preferred choice for the Aprilia (an earlier model non ABS bike that is) and of course the Storm.

* you may remember the boiling points I quoted as been ‘dry’, brake fluid boiling point values are quoted both wet and dry, wet is usually much lower then dry. Brake fluid is hydroscopic meaning it absorbs moisture from the atmosphere so the dry value is measured when the fluid is brand new straight from the bottle the wet value refers to the fluid's boiling point after absorbing a certain amount of moisture. The fluid starts absorbing moisture as soon as it is exposed to the atmosphere and continues to do so throughout its service life, which in effect means it is degenerating from day one, so it’s vital to change the fluid regularly maintain an acceptable boiling point. This is something that is often negected and should be the first thing on the list when looking at brake upgrades or dealing with brake performance issues.

[tony.mon]

Good and clear information, thanks for that

[lloydie]

Interesting read ,thank you .

[Jamoi]

Interesting rear ,thank you .

[Stephan]

I think we can easily use both, DOT4 and DOT5.1

I used TRW DOT5.1, but after bit of research, ATE Typ 200 seems to be the best option and fairly priced. Second is Motul RBF. Both have higher wet boiling point than any other performance brake fluid on the market. Except of Castrol SRF, which purely on racing level, and price is much higher.

The most important is to change the fluid once a year, I never discovered problem with TRW and ATE said above.

[AMCQ46]

good info budd. I hadn't twigged that DOT5.1 was a closer relation to DOT4 than it is to DOT5......... crap naming convention, they should have called it 4.1!!!!!


so can I mix DOT4 and DOT5.1 on my car? it says it should be DOT4, but I have to bleed the system through and only have DOT5.1 in my stores

[sirch345]

Thanks for posting that Budd, an interesting read. I agree with Al, they don't make things easy with their naming.

I stick with DOT4. I believe in changing the fluids on a regular basis due to it's hydroscopic nature as you mentioned, that is probably the most important thing,

Chris.

[budd]

good info budd. I hadn't twigged that DOT5.1 was a closer relation to DOT4 than it is to DOT5......... crap naming convention, they should have called it 4.1!!!!!


so can I mix DOT4 and DOT5.1 on my car? it says it should be DOT4, but I have to bleed the system through and only have DOT5.1 in my stores

Yes Alistair no problem mixing Dot 4 and 5.1, and it is a very poor way of distinguishing the different grades/types, the current labelling leads to the assumption that dot 4 is better than dot 3 followed by dot 5 then dot 5.1 in order of quality / performance but in fact dot 5 is a totally different composition so unless you read the small print on the packaging there is nothing obvious to suggest they are absolutely incompatible.

[AMCQ46]

cheers mate, I will get on with swapping pads and disks on the Cooper S now I know I can use my existing [only opened 1 month ago] bottle of 5.1.

[popkat]

Good post.
what about mineral oil, it's used in BMW and KTM Hydraulic clutches and bicycle brakes, not sure if it's used in motorcycle brakes though ?

Here's even more info on brake fluid as well as mineral oil, fairly long read though.
http://www.epicbleedsolutions.com/blog/ ... neral-oil/


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