Brake Fluid 101

I was driving dad’s car hauler through Smuggler’s Notch last fall bringing a car back from Stowe when I got a sobering lesson on why it is important to replace brake fluid. As I was passing the waterfalls in the Notch, the sedan with out of state plates in front of me abruptly stopped in the middle of the road to take a picture of the waterfall. I stomped on the brakes and came to an abrupt stop. Annoying, but no big deal. Moments later as I was descending the Jeffersonville side of the notch I tapped the brake as the truck started to run away at the top of the hill. No brakes. I pressed harder and harder on the pedal, but the truck was still speeding up. I manually down shifted and continued to thump on the brake. White smoke was now pouring off both front wheels. Ever notice that there is no runaway truck ramp there? I was able to stop the truck eventually. I pulled off to the side of the road and allowed the brakes to cool. How did this happen? The truck had just had the front pads replaced. When I looked at the brake fluid instead of a clear-yellow color, it looked more like grape juice. My theory is this: The new pads were slightly wider, creating some drag and some heat. The pistons were recently pushed back in to the calipers to accept the new pads, dirty contaminated fluid probably caused some rust on the caliper bores, causing the pistons to stick a little. The heat caused the brake fluid to boil, reducing the ability of it to compress the pads against the rotors. I changed the fluid, and exercised the pistons in and out a few times. Something the shop that replaced the pads should have done.

I have been using DOT 5, or silicone fluid in the antique cars for years with good results. Silicone fluid does not absorb moisture, so is perfect for antique cars. In the last year I have done the brakes on two MG cars that had conventional fluid, and one that I had changed over to Silicone fluid in 1988. The two with the conventional fluid had considerable corrosion in both the wheel cylinders and in the master cylinder. One MG, the green 1949 TC had half of the wheel cylinder pistons actually frozen. This car has marginal brakes to begin with, with half the wheel cylinder pistons frozen it was simply unsafe. The other MG had functional brakes when it was in a bad accident in 1999. The fluid was obviously in need of replacement, because all of the pistons became seized in their bores, requiring a great deal of effort to remove. The MG with the Silicone fluid had no corrosion in the master cylinder or the wheel cylinders. This is the black 1955 that is undergoing a total restoration, so I went through the brake system replacing all the metal brake lines, rubber hoses and seals. I have heard the horror stories of how the silicone fluid dissolved rubber parts, but have never seen any evidence of that or read anything that suggests this is really possible. My guess is that these owners would have experienced this regardless of what type of brake fluid was in their car. If you use DOT 3 or DOT 4 in your antique car I suggest you replace the fluid every two years, or switch to Silicone fluid. One other benefit to Silicone fluid is that it will not harm your paint. This is a great bonus for those master cylinders that are hard to fill without spillage. Just remember the old Midas commercial slogan, “the most important part of your car isn’t what makes it go, it is what makes it STOP!”

Why Change Brake Fluid?

Copyright AA1Car Brake Fluid is a hot topic because most people don’t know why it should be changed. Did you know the average motorist who drives 10,000 to 15,000 miles a year uses his brakes about 75,000 times a year? Did you know that nearly half of all motorists in a recent Car Care Council survey said brake failure was their number one fear amongst driving emergencies? So consider this: After three years of service, the average boiling point of the brake fluid has dropped to a potentially dangerous level because of moisture contamination and may not meet minimum federal requirements for brake fluid. Probably half of all cars and light trucks that are 10 or more years old in the U.S. have never had their brake fluid changed. Yet in many European countries, regular brake fluid checks are required, and half of all cars routinely fail such tests. That’s a good case for changing brake fluid. REPLACE BRAKE FLUID Brake fluid is one of the most neglected fluid in vehicles today, yet is vitally important for safe driving. Consequently, professional technicians should be checking the fluid and recommending that the brake fluid be changed if it is contaminated. The issue is old brake fluid may not be safe if moisture contamination is above a certain level.

Brake Fluid Preventative Maintenance

Many experts have long recommend changing the brake fluid every year or two for preventative maintenance. Their rationale is based on the fact that glycol-based brake fluid starts to absorb moisture from the moment it is put in the system. The fluid attracts moisture through microscopic pores in rubber hoses, past seals and exposure to the air. The problem is obviously worse in wet climates where humidity is high. After only a year of service, the brake fluid in the average vehicle may contain as much as two percent water. After 18 months, the level of contamination can be as high as three percent. And after several years of service, it is not unusual to find brake fluid that contains as much as seven to eight percent water.

An NHTSA survey found that the brake fluid in 20% of 1,720 vehicles sampled contained 5% or more water! As the concentration of moisture increases, it causes a sharp drop in the fluid’s boiling temperature. Brand new DOT 3 brake fluid must have a dry (no moisture) boiling point of at least 401 degrees F, and a wet (moisture-saturated) boiling point of no less than 284 degrees. Most new DOT 3 fluids exceed these requirements and have a dry boiling point that ranges from 460 degrees up to over 500 degrees. Only one percent water in the fluid can lower the boiling point of a typical DOT 3 fluid to 369 degrees. Two percent water can push the boiling point down to around 320 degrees, and three percent will take it all the way down to 293 degrees, which is getting dangerously close to the mini-mum DOT and OEM requirements.

Dave’s Brake Fluid article will continue in the April Issue


Please email all inquiries to: Dave
or snail mail
32 Turkey Hill Road
Richmond VT 05477