Answers to Brake Fluid Questions

by Sean Cathcart and Steven Fooshee

Here's the deal with brake fluid: (from a mustang magazine)

DOT 3 and DOT 4 are glycol based and absorb moisture over a period of time. Absorbtion comes from master cylinder vent and smallm pores in your rubber hoses and seals. When moisture enters the system, internal corrosion increases and boiling temps drop. DOT 3 & 4 and hyroscopic; they attract to water. Leaving the cap off your master cylinder or brake fluid bottle can ruin your brake fluid overnight!

Bleeding the brakes gets rid of all this moisture, renewing boiling points and corrosion resistance.
EG: After 2 years of use, a typical brake system holds 8-10% water. This lowers the boiling point 25% of its original wet value. This moisture, along with high-brake sytem temps, turns the brake fluid acidic and corrodes the brake suystem parts from the inside out.

DOT 5 is silicone based brake fluid. silicone based fluid repels water, thus moisture will not mix with the fluid. Regardless of fluid types, moisture will enter the system. Thus, with silicone-based fluids, the moisture pools and becomes trapped in low points of the brake system, such as calipers, hard lines, etc. This water can freeze and burst lines, and you can imagine the poor braking when you have a chunk of ice blocking the flow of brake fluid. This water will still corrode from the inside out. Silicone fluid is more difficult to bleed, as the air in the sytem can foam. This air is compressible, which malso makes silicone brake fluid altitude-sensitive. It is also more expensive.

DOT 5 offers no greater protection than DOT 3 or 4, but it does offer a higher boiling point. Use DOT 5 for severe open track and road racing, for everyone else, DOT 3 or 4 is more than adequate. Also, with DOT 3 or DOT 4, you can very easily rid the system of moisture by annual flushing. Notes- You should never mix dot 3 or DOT 4 with DOT 5, but you can safely mix DOT 3 with DOT 4.

The gist? If you have a race-only car, use DOT 5. For everyone else, DOT 4 offers the best protection and the highest boiling point. Bleed once or twice a year, (and buy speed-bleeders: well worth is!), and you will be more than satisfied with your performance.

Any quality brand DOT 4 will do, but I also like Ford's heavy duty fluid: higher boiling point. Other brands have higher boiling points, (such as z-rated DOT 4), but are more expensive and generally not required on a street car.
-Sean Cathcart

DOT 5 silicone is intended for cars that sit for long periods. It's compressible (*spongy* brakes) but will not absorb water. Which brings up a point... DOT 3&4s' absorption of water is intentional. As bad as corrosion is from water in the fluid, it's worse from water that's sitting free.

DOT 4 is mineral oil based and won't absorb as much water from the atmosphere. I use Castrol LMA in all my vehicles (bikes and Brit cars require DOT 4 due to seal mat'ls) and I highly recommend it.
-Steven Fooshee

written by: Sean Cathcart and Steven Fooshee