DOT 4 brake fluid: standards, boiling points and when to change it
DOT 4 brake fluid is specified for a great many cars – but what do DOT, FMVSS 116 and ISO 4925 Class 4 actually mean? This page explains dry and wet boiling points, how DOT 4 LV differs and why brake fluid has to be changed at regular intervals.
DOT 4 brake fluid: what the standards mean
DOT refers to the US Department of Transportation. Its safety standard FMVSS 116 defines performance classes for brake fluid: the glycol-based DOT 3, DOT 4 and DOT 5.1, and the silicone-based DOT 5. The classes differ mainly in their minimum boiling points and in the viscosity permitted at low temperatures.
Alongside FMVSS 116 there is SAE J1704 from the Society of Automotive Engineers and the international standard ISO 4925, which divides brake fluids into classes; DOT 4 largely corresponds to Class 4. The label of Brake Fluid DOT 4 states “DOT 4, exceeds DOT 3”, FMVSS 116, SAE J 1704 and ISO 4925 Class 4.
| Class | Base | Dry boiling point (min.) | Wet boiling point (min.) | Elat product |
|---|---|---|---|---|
| DOT 3 | Glycol | 205 °C | 140 °C | – |
| DOT 4 | Glycol | 230 °C | 155 °C | Brake Fluid DOT 4: label states above 260 °C and above 160 °C |
| DOT 5.1 | Glycol | 260 °C | 180 °C | – |
| DOT 5 | Silicone | 260 °C | 180 °C | – |
In brief
DOT 4 is a performance class for glycol-based brake fluid under the US standard FMVSS 116; internationally, it largely corresponds to ISO 4925 Class 4. Because brake fluid absorbs water and its boiling point drops as a result, it is changed at the interval the vehicle manufacturer sets – often about every two years. According to its label, Elat Brake Fluid DOT 4 has a boiling point above 260 °C and a wet boiling point above 160 °C.
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Dry and wet boiling point: why brake fluid ages
Glycol-based brake fluid is hygroscopic: it absorbs moisture from the air through hoses, seals and the reservoir vent. The more water it contains, the lower its boiling point.
The dry boiling point applies to fresh fluid with no water in it. The wet boiling point is measured in the laboratory after a defined uptake of water and shows how much reserve an aged fluid still has. For DOT 4, FMVSS 116 requires at least 230 °C dry and 155 °C wet; according to its label, Brake Fluid DOT 4 has a boiling point above 260 °C and a wet boiling point above 160 °C.
Things become critical on long mountain descents, when towing or under repeated hard braking: the fluid heats up, and vapour bubbles can form in fluid that contains water. Unlike liquid, vapour can be compressed, so the pedal goes soft or sinks to the floor. Water also promotes corrosion in callipers, brake lines and the ABS/ESP hydraulic unit.
Elat brake fluid to DOT 4
Every label entry matters for this brake fluid: the DOT class, FMVSS 116, SAE J 1704, ISO 4925 and both boiling points.
Brake Fluid DOT 4
DOT 4 · Brake Fluid
Specified to ISO 4925 Class 4; if DOT 4 LV or Class 6 is required, please contact us.
Also in our range (details on the product pages): Dot 4 LV ES.6 Class 6
Approval or specification?
Only expressly stated manufacturer approvals are marked “Approval”. All other entries are specifications and recommendations as printed on the product label; the entries relevant to this topic are highlighted. For your vehicle or machine, the owner’s manual and the manufacturer’s approval lists always take precedence. If in doubt, we are happy to advise you.
DOT 4, DOT 4 LV and ISO 4925 Class 6 compared
Some vehicle manufacturers call for a brake fluid that stays particularly thin even in severe cold, for example to suit their ABS and ESP control systems. Such fluids are usually labelled DOT 4 LV (low viscosity) and are often mentioned together with ISO 4925 Class 6 – a class with lower cold viscosity and higher minimum boiling points than Class 4.
According to its label, Brake Fluid DOT 4 is specified to DOT 4 and ISO 4925 Class 4 and is not an LV fluid. If your vehicle manufacturer requires DOT 4 LV or ISO 4925 Class 6, use a brake fluid that states exactly that. Our range also includes Dot 4 LV ES.6 Class 6; we are happy to check with you whether it suits your vehicle.
One rule applies across the board: glycol-based fluids and silicone-based DOT 5 must not be mixed. The label states that the Elat fluid exceeds the DOT 3 requirements; whether it may be used in a vehicle that specifies DOT 3 is confirmed by the owner’s manual.
Brake fluid change: interval and workshop practice
Because brake fluid absorbs moisture even in vehicles that are rarely driven, many service schedules set the change on a time basis. The interval is often about two years; the service schedule of the vehicle in question is decisive.
- Measure the condition: a boiling-point tester shows how far the boiling point of the fluid in the vehicle has already dropped.
- Use fresh fluid only: open containers immediately before use and reseal them tightly straight away; never reuse drained fluid.
- Follow the specification: take the DOT class and, where given, the ISO 4925 class from the owner’s manual – especially where LV is required.
- Bleed thoroughly: displace all the old fluid at every wheel brake. For some vehicles with ABS/ESP, the manufacturer prescribes a specific procedure or a diagnostic tool.
- Include the hydraulic clutch: if the clutch is fed from the same reservoir, it is part of the change.
- Collect old fluid separately: dispose of used brake fluid properly and do not mix it with other fluids.
From workshop practice
Note the date of the last change where it is easy to see, for example in the service book or on a sticker near the reservoir. The next workshop can then tell at a glance when the change is due.
DOT 4 brake fluid for workshops, retailers and fleets
Workshops, retailers and fleets with in-house maintenance obtain Brake Fluid DOT 4 directly from Global Tech Lubricants – if required, together with engine, gear and hydraulic oils and coolant from the Elat range. All label details are on the product page; further brake fluids are listed under brake fluids in the catalogue.
When you send an inquiry, include the required DOT and ISO class and the quantity you need. We supply business customers throughout Germany; for the range and all topic pages at a glance, see lubricant supply across Germany. If you are changing the brake fluid during a service anyway, it is often worth checking the coolant too – see our coolant standards page.
Frequently asked questions
How often should brake fluid be changed?
The interval is set by the vehicle manufacturer in the service schedule; it is often about two years. Because brake fluid absorbs water from the air, it ages even when a vehicle covers little distance. A boiling-point test in the workshop shows its actual condition.
What is the difference between dry and wet boiling point?
The dry boiling point applies to fresh fluid without water, the wet boiling point to fluid after a water uptake defined in the laboratory. For DOT 4, FMVSS 116 requires at least 230 °C and 155 °C respectively. The label of Elat Brake Fluid DOT 4 states above 260 °C and above 160 °C.
What is the difference between DOT 4 and DOT 4 LV?
DOT 4 LV (low viscosity) refers to DOT 4 fluids that remain particularly thin in the cold; they are often mentioned together with ISO 4925 Class 6. Some vehicle manufacturers require them for their control systems. Brake Fluid DOT 4 is specified to ISO 4925 Class 4 and is not an LV fluid.
Can I use DOT 4 instead of DOT 3?
According to its label, the Elat brake fluid exceeds the DOT 3 requirements. DOT 4 is permitted in many vehicles that specify DOT 3, but the owner’s manual is binding. We can check the requirement with you if you are unsure.
Can DOT 4 be mixed with DOT 5?
No. DOT 5 is silicone-based and must not be mixed with glycol-based brake fluids such as DOT 3, DOT 4 or DOT 5.1. Always top up with the type of fluid the vehicle manufacturer specifies.
Related topics
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