Brake fluid refilling - the same DOT is a must ?

[dreja]

My brake fluid is at min value so I have to fill it up.

As I don't know the type (DOT) of the current fluid, I'd like to know which brake fluid ( DOT ) would be suitable for filling ?
Is it allowed to add a different fluid ?

[Mux213]

Hey Dreja,

If you do not know the DOT I presume you have never changed the brake fluid. If so its no luxery to change the brake fluid completely some day anyways so I would invest in that some day anyways. If you use your car normally DOT 3 or 4 is fine and thats not expensive. Just make sure you bleed the car good or have it done by a professional.

As for mixing DOT, there are only some types of brake fluids you shouldn't mix, those are mostly special purpose ones that contain some chemicals that don't react well together (I believe DOT 5 is one of these but don't hold me to it, I would mix DOT 5 and DOT 3/4 anyways even if it were possible).

I doubt your car has anything other then DOT 3 or DOT 4 in there. DOT 3 is your run of the mill brake fluid that you find in any other car, DOT 4 is a little bit better. Unless the manufacturer specifies differently, there should be no problem in mixing these.

I'm sure someone here can give a bit better description of which DOT types there are and which is exactly for what, I always mix them up. I remember there being one DOT type you should stay away from, but can't remember which one it is.

[EE80Liftback]

I guess DOT4 would/should be standard in an environment that is less flat than the Netherlands, like Slovenia.

And MUX is right, If you've never replaced it then definitly do so, brake fluid attracts water. So with old fluid, apart from worse braking you also have a higher risk of getting problems with calipers seizing because of rust.

[dreja]

Hi,

thanks for both answers

Two years ago I had the car at the mechanic who changed the rear brake calipers.
At that time I all was focused on car renewal and passing the mot, so I've forgotten to ask him if they changed also the fluid as I ordered them.
So I'm planning to renew the fluid after some time, when I'll change the brake rotors and pads.

So as you say I'll try to buy a DOT 3 or Dot 4 and fill it to the maximum. And when I'm going to change the pads&rotors I'll definitly change also the fluid.

[assassin10000]

DOT 3 - Ok, stock stuff.

DOT 4 - Good/Better. Higher flash (boiling) point, good upgrade. Works fine/well with our braking system.

DOT 5 - BAD, do not use. Silicon based, does not deal with water well at all. Can lead to air/water pockets which are corrosive in our brake systems, can lead to brake failure.

DOT 5.1 - Best. Really high boiling point.

Then there is the racing fluids. Motul Racing Brake Fluid 660 FTMFW.

Here's a good read.
http://www.afcoracing.com/tech_pages/fluid.shtml

Andrew

[NoHachi]

Exactly. Go for Dot5.1 if you don't plan on flushing it often.
Racing DOT4 can sometimes outperform DOT5.1 for flashpoint but degrades quicker. Some good and cheap options out there for racers among DOT4.

[Rasteirinho]

Racing brake fluid is normally DOT4.

[SekiguchiUeno]

i thought:
Dot5.1 is for racing with higher boiling point and must be renewed often because it degrades quickly while it attract water.
Dot4 is daily use proven with lower boiling point bud higher durability over age. so which opinion is now correct?

[NoHachi]

Thats not correct.

The DOT number is determined by three factors:
1. Composition of the fluid (Only thing you need to know is that DOT5 is glycol based and does not mix with other DOT's. DOT5.1 does through, irritating naming convention that tends to confuse people).
2. Dry boiling point
3. Wet boiling point

Dry boiling point is for fresh fluid. Wet is the boiling point after the fluid has attracted x% of water (as it does in the brake system after a while).
The DOT standards are determined by both a minimum dry and wet boiling point. (see graph: dry is start of line, wet is at x%).
For DOT5.1 these are both higher then the DOT4 standard. Aka, DOT5.1 has a higher minimum dry boiling point AND it degrades less quickly the DOT4. They are however minimum standards.
This means you can have exceptions. For example:
A fluid that degrades very slowly over time might have a wet boilingpoint that meets the DOT5.1 standard. But if it doesn't have a high enough initial dry boilingpoint it is classed as a DOT4 (very rare situation).

Alternatively, some DOT4s have VERY high dry boiling points, but do not meet the DOT5.1 standard for the wet boilingpoint (they degrade quicker). These have to be called DOT4 but are fine for racing, where we sometimes change the fluid each session and don't care about the wet boiling point.
Hope this makes some sense, I'm sure there is enough real info out on the web.

In short: If you are not racing and switching fluid often, DOT5.1 is your best bet and is what you should be using if you care about performance.

[SekiguchiUeno]

Nohachi thanks for very usefull info, it verify what i know about brake fluids from my DH and MX times.
i know Dot5 has another consistecy and cannot be mixed,
and i know dot 5.1 is the better solution on enduro motorcycles and downhill bikes (only mixed it little bit over the age).
now i know why