Brake fluid Wet or Dry boiling point, which is more important?

Written by Jeff Ritter

Customers frequently ask us, "What is the best brake fluid for my application?" The discussion inevitably turns to wet vs. dry boiling points, and which is more important. Our technical director, Mike ONeil, summed it up nicely as follows:

Brake fluid bottles typically list two different boiling points, “wet” and “dry”. Yet, they do not mention which, if either is more appropriate. The short answer for racers is: those who flush their fluid often should pay much more attention to the dry boiling point.

Here is why:The wet boiling point might be more appropriately called a “very wet” boiling point. Yes, brake fluid is hygroscopic, meaning that it tends to absorb moisture from the air to which it is exposed. But, the rating is based on about 3% water content in the fluid. If you live in an area where “muggy” is a common term, do not have bellows in the reservoir separating the fluid from the outside air, and you have not cracked open a bleed screw in two years, you might have a 3% water concentration. The wet boiling point is more important for OE applications where little attention is ever paid to the brake system once the car leaves the dealership.

Most racers flush, or at least bleed, their brakes much more frequently however. The system is never totally free of water. However, the amount of water in the system is much less than 3%. Think more along the lines of 0.3%. If the dry boiling point is 600 F and the wet boiling point is 400 F, then the recently flushed boiling point is likely around 580 F, which is much closer to the dry boiling point.

If a reservoir does not have a set of bellows and the car is going into storage for more than a month, one should seal off the reservoir cap vent. A rubber band and plastic bag work well and are easily identified as needing to be removed before driving. This reduces water absorption. Otherwise, proper bellows do a good job on their own.

In summary, if you're an enthusiast who drives your car hard, and you're bleeding your brakes regularly, then a high dry boiling point is what you should be looking for in brake fluid.