Can someone give me a history on dot brake fluid?

[burnicarus]

Im working on my brakes replacing lines and a cylinder and also bleding them and a while ago knowing this was coming up I got dot 5.1 brake fluid. My system says dot 4 and after reading up I now know that there must be some big difference between the dot fluids. sorry that I sound ignorant of this but what do the dots refer to and why is it that you cant mix different dots? thanks

 

[dualitymike]

DOT 3 and DOT 4 are glycol based. The only difference between the two is purity - DOT 4 has a lower water content than DOT 3. DOT 5 is silicone based. You can't mix DOT 5 with anything DOT 4 or lower.

 

[locOleoN]

Sure..

Once upon a time there was DOT 1 break fluid.
Then came along DOT 2
Shortly thereafter DOT 3 was born...
A few years later, DOT 4 made a grand appearance over throwing DOT 3

Now we have DOT 4...

And everyone lived happily ever after...

The end

 

[burnicarus]

clever. Ive been reading and other than dot 5 they all seem to be interchangeable to an extent, the extent being that it will lower the grade of say dot 5.1 if its mixed with dot 4. If im wrong please tell me but Ive come to the understanding that it wont affect my brakes if I were to change over to 5.1 from 4.

 

[locOleoN]

Why complicate matters?

If the shop manual or the cap on the master cylinder says DOT 3 then use DOT 3 and move on!

 

[Scruffy]

Mixing DOT 5 silicone based with DOT ANYTHING ELSE gets you brake lines full of Jello and calipers full of clearish-yellowish-greenish-purplish crystals. I may still have an OOPS wheel cylinder off my truck, put DOT 5 in it 20 years ago by mistake... kept a wheel cylinder (4 wheel drum brakes) as a don't do this teaching prop.

If you want to use 5 instead of old fashioned 3 or 4, flush the system with denatured alcohol, then add the 5 and bleed as normal.

 

[harrydahl]

Dot 5 (silicone) is great for normal motoring, not for track use. Does not work well with ABS or servo boosters.
Dot 5.1 is the same as Dot 3 and 4, but meets another (normally higher) specification. Some older rubber parts , made for Dot 3, do not tolerate Dot 4 or 5.1

 

[crazypj]

If your changing everything don't worry.
DOT 5.1 is glycol bases but has a much higher dry boiling point and a higher 'wet' boiling point but it does absorb water faster than DOT 3 or 4 ('Wet' is 10% max water absorption - fluid turns 'mud' colour)
Realistically, DOT 3 is outdated use DOT 4 which is what your bike calls for.
DOT 3, 4, 5.1 can be mixed without any problems. DOT 5 silicon fluid doesn't mix with anything except some of the fully synthetic DOT 5 fluids which say they are compatible with DOT 3, 4, 5.1 (although I haven't tried and don't intend to)
From everything I've read about DOT5, it sucks for a 'daily driver' moisture will get into system and will collect at lowest point (also from experience, my CB550F1 needs new pistons and possibly calipers after filling with 'purple juice' )

 

[burnicarus]

Yea I learned that, replaced brakes with 5.1 although there may be a smidge of dot 4 left in the system I wasnt able to fully get out. Please dont misunderstand Iwas never considering using dot 5 I was talking about 4 and 5.1

 

[crazypj]

Don't worry about it until next year

 

[CarbsAndCylinders]

Yeah, what I have been told, although I have not researched it myself, is that DOT 5.1 is an evolution of Dot 4 and not like Dot 5. Also, Dot 5.1 is less viscous than Dot 5 so it will lighten brake lever feel and make it easier to modulate brake lever.