DOT 4 LV brake fluid: what to top up with and when to change it?

You know the scene: at a service the mechanic points at the reservoir, where the brake fluid is down to the lower mark, and advises you to top it up. In the shop the shelf holds DOT 4, DOT 4 Plus, Super DOT 4, DOT 4 LV and DOT 5.1, and the salesman assures you that “it’s all the same thing, just take any DOT 4”. Not quite. Brake fluid is one of the few fluids in a car that decides not how long some component lasts, but whether the car stops at all. And now it has a new variety as well.

Put simply, brake fluid carries the force from the pedal to the brakes at the wheels, and it must neither boil in the heat nor thicken in the cold. Hence the requirements placed on it.

What DOT 3, 4, 5.1 and 5 mean

DOT is neither a brand nor a manufacturer, but the abbreviation of the US Department of Transportation, whose standard FMVSS 116 sets the minimum requirements for brake fluids; the international standard ISO 4925 has the corresponding classes 3, 4 and 5.1. As with oil classes, the number on the can is a set of requirements, not a recipe (we have already covered this in the article on analogues and substitutes).

There are two main requirements: the boiling point and the viscosity in the cold. The boiling point is tested twice — for “dry”, that is fresh, fluid and for “wet” fluid that already holds about 3.7% water:

DOT 3, DOT 4 and DOT 5.1 are relatives: all three are glycol-based and mix with one another, and here a higher number means stricter boiling-point requirements. DOT 5, on the other hand, is a completely different, silicone fluid, and it must never be mixed with the glycol ones: you will not get a uniform mixture, and any moisture that gets into the system collects separately and corrodes the metal. Do not smile — thanks to a dot and a one, people really do confuse DOT 5 with DOT 5.1.

And let me say straight away: “a higher number” does not mean “better for your car”. In the cold, DOT 4 may by the standard even be thicker than DOT 3. Only the car manufacturer knows which fluid the brake system was designed for.

Class 6 and DOT 4 LV

Now look at the last column of the table. Ordinary DOT 4 is allowed to thicken in the cold to 1,800 mm²/s. But a modern car does not brake by the driver’s foot alone: ABS and the stability control system (ESP or ESC) release and build up the pressure at each wheel on their own, very quickly, pumping the fluid through valves with fine channels. Try squeezing honey out of a syringe through a thin needle in winter — that is roughly how an ABS unit feels with thickened fluid in the cold.

That is why the ISO 4925 standard also has class 6. In essence, it is a glycol DOT 4 whose viscosity at −40 °C is no more than 750 mm²/s — less than half of what ordinary DOT 4 is allowed — and whose boiling point is even slightly higher: at least 250 °C “dry” and 165 °C “wet”. There is a class 7 as well: the same low viscosity, but with boiling points at DOT 5.1 level. Many new European cars are filled with class 6 fluid at the factory.

Most often such fluid is labelled DOT 4 LV (from low viscosity); you will also come across names such as DOT 4+ or Super DOT 4. But bear in mind that nobody protects these names, and plain DOT 4 with a higher boiling point may be called a “plus” too. So look on the label not for a word but for the class: “ISO 4925, Class 6” or DOT 4 LV.

But before you top anything up, find out why the level has dropped. Most often the pads have worn: the caliper pistons have moved further out, and the fluid from the reservoir has followed them. Once the pads are replaced, the level comes back by itself, so do not fill the reservoir to the brim. And if the pads are still thick but the level keeps falling, look for a leak — and not in a shop, but at a service station.

So what should you top up with?

1. What the car manufacturer requires. If the service book or the owner’s manual says DOT 4 LV or class 6, top up with exactly that. How the story with the first brake fluid that came to hand at a car market ended is told in the article on which service fluids to buy.

2. Ordinary DOT 4 mixes with DOT 4 LV, as both are glycol-based. But the mixture will no longer be class 6: in the cold it will thicken more than the ABS designers counted on. As a way out on the road it will do, but at the first opportunity it is better to change the fluid completely.

3. DOT 4 LV can, as a rule, also go where the manufacturer requires ordinary DOT 4, provided the label states DOT 4 compliance: it meets the DOT 4 requirements with a margin, and is thinner in the cold into the bargain.

Why the fluid has to be changed even if the level never drops

Glycol brake fluid is hygroscopic: little by little it absorbs moisture from the air — through the vent in the reservoir cap, through the rubber hoses, every time the cap is opened. The level does not change because of it, but the boiling point falls: by the standard, fresh DOT 4 must boil at no less than 230 °C, while with about 3.7% water the limit drops to 155 °C.

What that threatens is easy to picture: long braking on a descent => the calipers get red-hot => the water in the fluid boils => vapour bubbles appear in the system => vapour, unlike fluid, compresses => the pedal goes to the floor. And it happens not in a car park, but exactly when the brakes are needed most. On top of that, water slowly corrodes the calipers, cylinders and the ABS unit from the inside.

That is why the brake fluid change interval is counted in years, not kilometres: as a rule, once every two years, regardless of mileage, and the exact interval for your car is in the service book (see the note change all the fluids). A car that has stood in a garage for two years needs a change just the same: the fluid takes on moisture with time, not with kilometres. And the colour tells you nothing here — fluid with a dangerous amount of water can look perfectly fresh. Only a tester at a service station will show the water content or the boiling point.

So, brake fluid is not chosen by the bigger number, and it is not topped up instead of being changed: you fill it with the one the car manufacturer requires and change it at the interval in the service book. And if a change seems a needless expense, find out what a new ABS unit costs, for comparison.

Further reading:

• What are “original” service fluids?
• Why does the colour of antifreeze tell you nothing?