DOT 3 vs DOT 4 Brake Fluid #OilChat 85

brake fluid
brake fluid

The U.S. Department of Transportation (DOT) classifies brake fluid into four main categories i.e. DOT 3, DOT 4, DOT 5 and DOT 5.1. The primary differences are their composition and boiling points. (We have discussed brake fluid in detail in OilChat 27 and suggest you revisit it to refresh your memory). Discussions in this newsletter will be restricted to DOT 3 and DOT 4 brake fluids.

Since 2006 DOT 4 is the most common brake fluid used in cars and light commercial vehicles. This is due to higher brake system temperatures, as well as widespread use of anti-lock braking systems (ABS), Electronic Stability Programmes (ESP) and traction control systems (TCS). Nonetheless some (mainly older) vehicles still require DOT 3, but since it is no longer readily available the question is often asked whether one can use DOT 4 instead of DOT 3 brake fluid. Unfortunately there is no straightforward answer to the question.

Both DOT 3 and DOT 4 brake fluids are glycol-ether based and therefore these fluids can be mixed without compatibility issues. Brake fluids are exposed to very high temperatures during braking and and the U.S. Department of Transportation have therefore included minimum boiling points in their Federal Motor Vehicle Safety Standards (FMVSS) 116 brake fluid specifications:

FMVSS 116 brake fluid specifications

There are no standard formulations for brake fluids, but DOT 3 generally includes about 80% glycol-ether while DOT 4 typically has 50 to 65% glycol-ether with 20-30% borate-ester to improve the high temperature characteristics of the fluid. And this is the reason for the confusion.

Older vehicles that are still on the road today may have had their brake systems designed before DOT 4 brake fluid was introduced. Some brake hoses used in these vehicles, particularly those with inner tubes made of SBR rubber, were found to be incompatible with certain DOT 4 formulations in laboratory testing. The suspect brake fluids appear to be ones with high borate-ester content. It is believed that these formulations permeate the inner tube and then react with the PVA reinforcement braiding to produce a viscous liquid which could build up between the layers of rubber and make the hose considerably weaker. Attempts to reproduce this problem in real life conditions have proven to be difficult though. Most vehicle manufacturers today, however, use a different rubber (EPDM) in their brake hoses which is much more resistant to permeation.

There are several 2006 and older vehicles on the road today that still operate perfectly with their original brake hoses, but when considering what can happen when brakes fail, it is better to be safe rather than sorry. Standard rubber brake hoses do not cost a fortune and are considered consumables, i.e. they need to be replaced. Most vehicle manufacturers recommend you do so at least every six years. This implies that the original brake hoses still fitted to 2006 and earlier vehicles are long overdue.

IMPORTANT: Glycol-ether based brake fluids are hygroscopic which means they absorb moisture from the atmosphere and need to be replaced at least every two years – see your vehicle owner’s manual. It is also important to remember that brake fluid is toxic and combustible and can damage the paintwork of your vehicle.

Boiling point for FMVSS 116

Blue Chip DOT 4 Brake Fluid is compatible with all materials used in the brake and clutch systems of late model vehicles and exceeds the boiling point requirements of FMVSS 116 as shown below:

If you have any questions about brake fluid, you are welcome to email us at and one of our technical experts will respond to your query.