What’s the right brake fluid for my car?

Question:

Is it possible to predict the compatibility of brake fluids with different materials used in brake cups and O rings? A hydraulic engineer told me that various components require different types of fluids. For example, he said that tanks and airliners require fluids that will not burn. I have used various combinations of fluids and parts that have worked perfectly, even in long-term storage.

However, one combination would not allow the master cylinder to move at all when bench tested. Jaguar advises to never use silicone fluid. If there were charts around that covered brake fluid compatibility a lot of problems could be avoided.

Answer:

Stutz came out with hydraulic brakes back in the early ’20s, but their system used water for brake fluid. Many older European cars such as Citroen and Jaguar required mineral oil due to the composition of the rubber components in the system. Conventional DOT 3 and DOT 4 brake fluids will damage the rubber parts in such cars. Most American cars, new or classic, use DOT 3 or DOT 4 brake fluid, which is essentially glycol ether. The only problem with that type of fluid is that it absorbs moisture and should be changed every couple of years to prevent moisture buildup and corrosion.

The alternative to glycol ether products is DOT 5 silicone fluid which was developed for the military and postal service. It is a bit more expensive, but it does not absorb moisture and it does not need to be changed frequently. It cannot be used in ABS systems, but for our classics it is great stuff. It is compatible with the DOT3 and DOT4 fluids, but if you mix them you sacrifice the anti-rust characteristics of the DOT 5 silicone fluid.

You can completely purge your old system of the glycol ether fluid, though, and make sure it is dry and then fill it with silicone fluid, or if you are installing a new brake system as part of a restoration, you can fill it with silicone fluid and bleed it the conventional way. Cars that I restored in the 1980s using silicone fluid have been in service for many years with no problems at all. The only caveat is if your car’s service manual or the instructions accompanying brake components you have purchased tell you to avoid silicone fluid, use whatever fluid they specify.