Brake fluids .... More useless information

There are two types - Synthetic (Polyglcol ether based ) and Silicone types. THEY WILL NOT MIX (honest)

the designations are as follows:

DOT 3 - Polyglcol Synthetic - this is a conventional fluid in general use
DOT 4 - Polyglcol Synthetic - similar to DOT 3 but better qualities regarding oil absorption and a higher boiling point.
DOT 5.1 - Polyglcol Synthetic, as DOT 4 but even higher boiling point!

DOT 5.0 is the Silicone based Fluid - Beware !

Basically Polyglycol fluids are the norm - available widely. You can mix DOT 3/4 and 5.1 fluids ( although not recommended as it can compromise the benefits of the higher spec fluid )

Polyglycol fluids are Hygroscopic - this means they absorb water which is one of the reasons your brakes can go spongy and you should change your brake fluid at least every two years - also the water could cause internal corrosion or your braking system. PAG fluids will also damage paint so be careful.

Silicone fluids - basically don't know why you'd use these - they can give you spongy brakes even with new fluid, have lower boiling points than DOT4 fluids, then again, they don't ruin your paint if you're a clumsy type, they don't absorb water (although any water collecting in the fluid will "puddle" causing corrosion) Silicone fluids normally have a purplish tinge to them where Polyglycol types are more straw coloured

If you change from one type to the other, (Polyglycol to Silicone, NOT DOT3 - DOT4 / 5.1 ) you'll need to completely flush and clean the system and best to change all the seals at the same time.

For everyday use, DOT 4 is fine, for racing or really heavy brake usage; go for a high boiling point such as Super DOT 4 or even a race fluid like Castrol SRF Racing brake fluid (similar properties to DOT 4 but for use where extreme braking conditions are encountered.

Silicone fluids don't absorb water and don't ruin your paint - but they can cause water to puddle causing localised corrosion and you can get spongy brakes even with new fluid - and if they get water in you can get a vapour lock.

Boiling points – typical

DOT 3 245 degrees
DOT 4 256 degrees
Super DOT 4 280 degrees
SRF Racing 300+ degrees

Silicone fluids - 260 degrees


Hope that's useful?

You need to get out more.

Funny, you're not the first to say that today ...

Ta. I knew most of that already, but still good to see.

[Stu]

What's the Boiling point of 5.1?

Originally Posted by Stu

What's the Boiling point of 5.1?

270 C Dry, 191 C Wet (Absorbed 3.5% water)

[Stu]

Ah! another geek is outted!

Originally Posted by Stu

Ah! another geek is outted!

Google-fu

Originally Posted by Stu

What's the Boiling point of 5.1?

From looking at the web of various manufacturers - around 270 degrees

Dot 5.1 are normally lower viscosity when cold ( thinner ) so tend to be recommended for cars with ABS in cold climates because they will be thinner when cold, but in the UK many will use DOT 4 with no issues

Another interesting fact is that Polyglgol fluids are 2x less compressible than silicone fluids

Nana

[yorkie_chris]

So an old bike which specifies DOT3 will be alright to use DOT4 in?