[DarrenR]

just some added info... this is in reference to glycol based brake fluids only

DOT 3 or brake fluid that meets SAE J1703 is an older specification, DOT 4, DOT 5.1 are all glycol-based fluid and as per the standard are required to be compatible for use in brake systems that specify a DOT 3 or higher.

DOT 4 or higher that meet the specification of SAE J1704 have a far higher boiling point than that of the lower J1703 standard. The downside of using a DOT 4 or higher brake fluid that ONLY meets the newer J1704 standard is the additive may be incompatible with some seals used in brake systems, the incompatibility is only so far as the seals deteriorate over a long period of time, but generally not within the time frame that a brake system would generally be considered in need of being overhauled.

A DOT 4 or higher specification brake fluid will also absorb moisture quicker than a DOT 3 specification brake fluid, but again not within the normal service life of brake fluid, which is considered in the industry and manufacturers as a period of 2 years. The Australian Standards for the maximum moisture content in a brake system is 4%, again this generally will be that 2 year time frame.

So consider this...
*A brake fluid that is DOT 4, DOT 5.1 has a higher boiling point.
*A brake fluid that is DOT 4, DOT 5.1 have better corrosion inhibitors than lower spec fluids.
*When buying a DOT 4, DOT 5.1 look on the label for the standards of BOTH SAE 1703, SAE 1704 and you are guaranteed it will be compatible with your brake system.
(Often a DOT 4 brake fluid that meets both standards will be labeled as Super DOT 4 or DOT 4 plus)
*Regardless what you use, flush your brake fluid every 2 years.
*Brake fluid is hygroscopic which means it absorbs moisture, from the moment the container is opened AND when it is in the brake system of a vehicle. Never use opened brake fluid that has been sitting on the shelf for any period of time, buy small 500ml containers and when you have finished discard un-used fluid.
*Brake fluid is extremely toxic to you, any living plants or animals, disposing in the bin is not an exceptable method of disposal and poses a health risk and an environmental hazard.


Important to note:
Some vehicles and heavy equipment (common on French manufactured vehicles/equipment) use a mineral or silicone based brake fluid, DOT 5 (please note this is NOT the same as DOT 5.1) is a silicone based brake fluid, the other being "LHM" is a mineral based brake fluid.

Silicone or mineral based brake fluid must never be mixed with "normal" glycol based brake fluids.

Regards
Daz