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How are brake fluid types classified?
Brake fluids are hydraulic oils specially formulated for the use in automotive braking and clutch systems.
Although a few standards do provide classifications for the different types of brake fluids, the most used standard by far, remains the American FMVSS Standard No. 116 standard of the Department Of Transportationn - DOT.
For this reason, the cap of your brake fluid reservoir will almost certainly refer to the use DOT 3 (older vehicles) or DOT 4 (recent vehicles) and perhaps DOT 5.1, rather than any other standard.
The below explains how and when to use each, the equivalency and compatibility between the brake fluids, and which one to use in your vehicle.
Although a few standards do provide classifications for the different types of brake fluids, the most used standard by far, remains the American FMVSS Standard No. 116 standard of the Department Of Transportationn - DOT.
For this reason, the cap of your brake fluid reservoir will almost certainly refer to the use DOT 3 (older vehicles) or DOT 4 (recent vehicles) and perhaps DOT 5.1, rather than any other standard.
The below explains how and when to use each, the equivalency and compatibility between the brake fluids, and which one to use in your vehicle.
Brake Fluids Grades Chart
You will find many charts explaining the chemical compounds serving as the base for each brake fluid type, advising on which ones can be mixed together, etc. thus it is important to highlight the facts and rules to strictly follow not to damage your braking system or put your safety at risk!
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So instead, we propose a modern indicative brake fluid classification chart as per the below, noting you should strictly follow the automaker recommendations:
DOT 3 |
DOT 4 |
DOT 5 |
DOT 5.1 |
Glycol-Ether based |
Glycol-Ether based |
Silicone based |
Glycol-Ether based |
Used on old cars, prior to the ABS system introduction (1988), thus suitable for older vehicles with no ABS. |
Used on most modern vehicles, with or without ESP. This remains by far the brake fluid with the widest range of applications. |
Very special uses, to be used only if and when imposed by the vehicle maker, especially such fluids are most found in military equipment. |
To be used strictly when imposed by the automaker, usually on some ultra-high performance vehicles. |
Can brake fluids be mixed or should you upgrade to the DOT with the higher number?
The immediate answer is NO to all.
NEVER mix brake fluids with only one exception: an emergency where you find no other choice, in which case you can add any of DOT 3, DOT 4 or DOT 5.1 to your fluid reservoir, so you can continue driving. You should then replace your brake fluid and bleed it as soon as possible, for the below reasons:
NEVER mix brake fluids with only one exception: an emergency where you find no other choice, in which case you can add any of DOT 3, DOT 4 or DOT 5.1 to your fluid reservoir, so you can continue driving. You should then replace your brake fluid and bleed it as soon as possible, for the below reasons:
Hydraulic oils (brake fluids) equivalency chart within automotive braking systems:
DOT 3 instead of DOT 4 |
NOT POSSIBLE |
DOT3 fluid has a lower boiling point that will reduce the braking performance of your modern car, put your safety at risk, eventually damage the calipers. |
DOT 4 instead of DOT 3 |
NOT POSSIBLE, except... |
DOT4 fluid is no upgrade to DOT 3 as it will provide no advantages of any kind on old vehicles. Its compounds could not be compatible with the rubber parts of your old or classic car, thus could damage them. Use DOT 4 on an old vehicle, only in case you'd have replaced the whole hydraulic braking system with new parts, recently built. This includes the master brake cylinder and the calipers. |
DOT 5.1 instead of DOT 4 or DOT 3 |
NOT POSSIBLE |
DOT5.1 brake fluid has a much lower viscosity than DOT3 and DOT4. Cannot be used in braking systems not designed for it. |
DOT 3 or 4 instead of DOT 5.1 |
NOT POSSIBLE |
DOT3 and DOT4 brake fluids have lower boiling points and almsot double the viscosity of DOT5.1, no possible use in a hydraulic braking system designed for 5.1 |