Should I use DOT 3, 4 or 5 brake fluid?

February 1, 2011 | By Richard Prince


I have a question about brake fluid for my cars from the 1950s. They are not driven every day and are stored in the winter. The master cylinder says to use only DOT 3 fluid, but I have heard a lot lately

about DOT 5 fluid being better because it does not absorb moisture like the DOT 3 and DOT 4 fluids do. Would I be better off using the DOT 5 fluid or is it not compatible with the older car braking systems?


DOT 3 and DOT 4 brake fluids are glycol based and as such they are hygroscopic, which means they readily absorb water. One of the strengths of glycol-based fluid is that the same characteristics that cause it to absorb moisture also enable it to disperse moisture, thus avoiding the formation of water droplets that can impair brake function.

Over time, however, enough moisture accumulates in the fluid to impair brake function and promote corrosion of brake system parts. DOT 3 and DOT 4 fluids are chemically corrosive and will harm certain plastics, paint, metals and other things they may come in contact with.

DOT 5 fluid is silicone based and, therefore, it does not absorb water. This is an advantage as long as the brake system is well sealed and no significant amount of moisture is allowed to get into the fluid. The same qualities that prevent DOT 5 fluid from absorbing moisture do not enable it to disperse water so if a significant amount of water makes its way into the fluid it will gather together to form droplets that impair brake function and promote corrosion.

Furthermore, silicone fluid is prone to aeration, making it more difficult to bleed all of the air out of it. Silicone fluid is also inherently more compressive than glycol fluid so brake systems using silicone fluid tend to have a slightly spongier feel to the brake pedal.

The main advantages of silicone include a considerably higher boiling point, making it better for applications where a great deal of heat is created in the braking system, and the fact that it’s not chemically corrosive.

Glycol and silicone fluids each have their own advantages and disadvantages so there is no clear answer to the question of which one is better for you. But if you do decide to switch your collector cars to silicone fluid there are a number of things you need to know and consider first.

Silicone and glycol fluids are not compatible so every trace of glycol fluid must be removed from the system. It is not sufficient to simply drain the old fluid out and pour the new fluid in. You should disassemble the master cylinder, brake calipers or cylinders, and any valves or switches in the system for cleaning. You should also put solvent such as lacquer thinner or brake cleaning fluid through all of the lines and then thoroughly blow out all of the lines with clean compressed air.

Older rubber hoses and seals may not be compatible with silicone fluid so it is advisable to replace all of the hoses and seals throughout the brake system if they are not relatively new.

When handling silicone fluid do not shake or agitate it because it is easy to introduce air bubbles into it. When manually bleeding a system with silicone fluid, pump the pedal very slowly and gently to avoid agitating the fluid and causing air bubbles to form.