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 Hydraulic Brake - Brake Fluid

The water content is plotted on the x-axis.

If you let experts explain how relatively easy it is to obtain new brake fluid from old one and how complicated this process is when the old one is contaminated by some substance, such as oil, then you would like to make more calls to garages and maybe even do-it-yourselfers, and ask them to hand them in separately and cleanly.

Within the brake system, the brake fluid has the decisive task of transmitting force. Not to be imagined, it would lose an important property of almost all fluids, namely that they are incompressible. Of course, it loses this ability when all or individual components of it are transformed into the gaseous state.

So the water content in the brake fluid has to be observed very precisely, this also because of possible ice formation. The pour point of brake fluid should therefore be lower than all temperatures to which the car can be exposed. The pour point of brake fluid must be at least -65°C.

But of course, with a brake system you have to be primarily concerned with the possible heat development. This is most pronounced on fixed caliper brakes, where the brake fluid also has to be on the outside of the disk. In general, the 100°C boiling point of water is reached more often, so that too much water would become a compressible component of the brake fluid.

A pedal step without effect would be the consequence. Even worse, a defect like that doesn't announce itself beforehand. And as is often the case with brakes, it occurs when the brake is most needed. The penetration of water into the brake system can't be avoided completely, because the pumping movement within the system repeatedly allows air with varying moisture content to enter, strangely enough sometimes more by the sealing and diffusion at the brake hoses than by the ventilation opening.

So a regular exchange is called for. This, by the way, also applies to the future electric vehicles that can be foreseen now, all of which cannot yet manage without hydraulics. Pure brake-by-wire is not yet in sight. Another advantage of changing the brake fluid is that all possible undesirable components in the system are also removed, such as rubber abrasion and metal particles that could disturb its proper functioning.

DOT 3  140°C  FMVSS 116
DOT 4  155°C  FMVSS 116
DOT 5  180°C  ISO 4925
DOT 5.1  175°C  ATE
DOT 5.1  180°C  ISO 4925

One tests the so-called wet boiling behaviour of brake fluid (table above) with a defined water content of 3.5 percent. Also because it is hygroscopic, i.e. likes to absorb water, a certain amount of water absorption cannot be prevented. Therefore the boiling behaviour with a defined water content is more practical than without. In the diagram at the top you can see the requirements which the different quality classes of brake fluid must meet.

Permitted minimum boiling point in Germany: 180°C.

Brake fluid consists of either 95 percent polyglycol compounds and sometimes silicone fluid. Both fluids must not be mixed. So the change from DOT 3, 4 or 5.1 to DOT 5 is taboo. Brake fluid must not only be stored waterproof or directly consumed completely, it is also toxic and must not get on the paintwork or must be removed directly. As already mentioned, there are also significant differences to engine oil.

Changing intervals for brake fluid are often not taken seriously, especially since many drivers often switch from a brand workshop to a free one, which may feel less bound by the official service plan, especially at the time when a lack of change can have a negative impact. How good that there are at least enough testers for brake fluid, some of them even affordable for the car driver himself.

Here is a professional device from ATE, which is immersed with its sensor in a sample, takes up some of the sample and determines the boiling point by heating. The boiling point is then not only shown on the display, but also printed out by a printer located above it.

Surely a sample from the brake fluid reservoir is sufficient, especially since you can add a small risk factor. But if you want to be on the safe side, you can apply pressure to one of the two front wheels. That's where it gets the hottest. One should not underestimate the dangers.

If steam bubbles form in one of the two circuits alone, stepping on a brake pedal with a different pressure point is so irritating that, depending on the speed, you can lose a few crucial metres of braking distance. With diagonal distribution, relatively evenly overheated front brakes could be affected almost simultaneously. With black and white distribution, the rear brake is not really suitable for feeling safe.

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