Brake fluid

[vito84]

Hi guys!
which kind of DOT do you use in a standard brake system?
Thank you

[LotusJoe]

Dot 4

[Brian]

Castrol GT LMA (low moisture avidity).  Great for cars like ours that are generally not used as daily drivers.  I believe the name has recently been changed......check Pegasus Racing.
BW
743402R

[BDA]

I believe they changed the formula of the standby, LMA, to a synthetic formula. I can't always find it locally, so I've been using Valvoline (which is a DOT 3&4 fluid - I'm not sure what they mean by that, but I suspect it's a marketing thing for people used to buying DOT 3 fluid) but Castrol (DOT 4) would be my first choice.

I have been warned (and I take the warning very seriously) not to use DOT 5 (silicone) fluid. It is even less likely to absorb water than Castrol LMA and has higher wet and dry boiling points than DOT 4) but I have heard that it is much more difficult to bleed. There is a DOT5.1 which has the same wet and dry boiling point as DOT 5 fluid but it has at most 70% silicone. I presume that is to correct some of the problems of straight silicone fluid (DOT 5).

For my Mercedes, I use ATE SL.6 which is a DOT 4 fluid and is supposed to be less viscous for the fine passages in ABS and traction control systems. I suspect that would make it easier to bleed.

I think as long as you get a reputable brand that is DOT 4, you'll be fine. For me, that's Castrol, Valvoline, and ATE.

[EuropaTC]

For my Mercedes, I use ATE SL.6 which is a DOT 4 fluid and is supposed to be less viscous for the fine passages in ABS and traction control systems. I suspect that would make it easier to bleed.

Oh, now that's interesting.  I've not heard of that concept before, is it just a Mercedes thing or does it apply to all modern cars with electronic wizardry ?  The idea of something which might be easier to bleed air from has obvious attractions    (...to someone as lazy as me    )  I presume if it's the same basic chemistry as dot4 then they are compatible if mixed ?

To the OP - I use basic standard dot 4 the same as everyone else.

The Dot 5 stuff was quite a craze with some classic car fans over here at one time, the idea of filling the system and never having to replace it because it didn't absorb water was the big selling point as I recall.   I never bothered because of the dire warnings of mixing Dot4 & 5 - goblins would descend on your house, your dog would get fleas and the mixture of dot4 & 5 wouldn't work properly - were all too much for me to cope with at the time.   

After a few years there were also tales of folks reverting to Dot4, claiming the difficulty in bleeding Dot5 and getting a solid pedal feeling.  Don't know if the latter is true or not, it was just the usual car-meet chatter.....

Brian

[jbcollier]

Two differences between DOT 3 and DOT 4:

DOT 4 has a higher minimum boiling point both when dry and "wet".

DOT 3 has less borate additive (raises boiling point) which can swell rubber.

https://en.wikipedia.org/wiki/DOT_4

We use almost exclusively DOT 3 in all our work, unless the vehicle specifies DOT 4.  The big secret?  Change it every two or three years and then the lower wet boiling point doesn't matter and your hydraulic parts won't corrode.

[Grumblebuns]

If one has the time, there is a loooong tech article by Michael Grant from Moss Motors on brake fluid and application to British cars. This is a small snippet from the article that may apply.

"Traditional Concerns for British Car Owners

In the good old days, dire warnings of problems caused by using the wrong brake fluid in a British car were common. Anyone reading a warning like "as the cups in the master cylinder are pure [natural] rubber; it is imperative to use only the recommended fluid. Any other fluid may be dangerous" would probably take it seriously. Such strong concerns were very valid in the 1950s, but much less so now, even for 1950's vintage cars. There are several reasons why we can be less worried about our hydraulic systems "turning to goo" if the wrong fluid is used:


1) Pure natural rubber hydraulic seals are no longer made for our cars
 2) More than likely, the original natural rubber seals have been replaced.
 3) Brake fluids meeting current standards are compatible with the virtually every type of seal available, including natural rubber
 4) Brake fluids meeting current standards are safe to mix (although mixing them is not recommended)"

This is the link to the article:

http://www.mossmotors.com/SiteGraphics/Pages/Brake_Fluid/brake_fluid_long.html
 

[BDA]

Oh, now that's interesting.  I've not heard of that concept before, is it just a Mercedes thing or does it apply to all modern cars with electronic wizardry ?  The idea of something which might be easier to bleed air from has obvious attractions    (...to someone as lazy as me    )  I presume if it's the same basic chemistry as dot4 then they are compatible if mixed ?

Well, you got me thinking about this some more. I had heard about the viscosity issue on a Mercedes forum I frequent so maybe it deserves a grain of salt, but it sounded reasonable. I did a little poking around on the interweb and found this:

Why measure the viscosity of brake fluids?
The safe function of the entire brake system depends on
the hydraulic fluid's viscosity. A too high viscosity at low
temperatures results in a delayed response or, in the
worst case, in a complete failure of the brake system. A
too low viscosity at high temperatures causes poor
lubrication; the brake system’s hydraulic cylinders suffer
premature wear and may even block. So brake fluids should have a high viscosity index, i.e.
their viscosity should not easily be influenced by
temperature changes.
Viscosity in general and especially the low-temperature
viscosity is an essential parameter for anti-lock brake
systems (ALB, ABS), antiskid systems (ASR) or
electronic stability programs (ESP).
In hydraulic clutches the fluid's viscosity influences the
return speed of the clutch pedal.

Mercedes is pretty particular about which fluids it approves and not every producer is interested in spending the time and money to get on the list (thus the Redline oil in my transmission is not approved), so there are probably many other producers that produce appropriate brake fluid. Now a days most modern cars seem to have much more complicated hydraulics so I would assume that a lower viscosity brake fluid would benefit all of them, but it could just as easily be that any or most modern brake fluids would be appropriate. Assuming that the ATE actually is less viscous than many other fluids, I'm drawing the conclusion that it might therefore be easier to bleed because I think viscosity and surface tension are generally related.

Viscosity is a property of brake fluid I had never thought of before. but given the simple brake systems on our Loti, I don't think it matters that much to us.