February 2010 Techline

A larger-bore master cylinder will push more fluid down the brake lines than a smaller-bore cylinder does, but keep in mind that the brake line and wheel cylinders can only accept a given amount of fluid in direct proportion to their given total capacity in cubic inches of volume. However, a larger-bore master cylinder will move more fluid than a smaller-bore unit, given the same amount of piston stroke, i.e., pedal travel. In other words, with a larger-bore master cylinder (say a 1-inch versus 7/8-inch diameter), the pedal requires less travel to move a given amount of fluid, but will require more pedal-pushing power to gain the amount of line pressure to stop the vehicle. The reverse happens when a smaller bore cylinder replaces a larger one. It takes less pedal-pushing power but more pedal travel, as the piston must make a longer stroke to move the same amount of fluid as in the larger-bore cylinder.

A classic example was when one put 11-inch brakes with their corresponding larger wheel cylinders on an early Jeep to replace the original 9-inch brakes, but retained the stock master cylinder. Instant power brakes! Both were due to the shoe size and to the change in master versus wheel cylinder ratio. The downfall was that it was necessary to keep the brakes fully adjusted. Otherwise, one could push the pedal dangerously close to the floor.

Brake fluid: Okay, what is the big deal about all the different brands of brake fluid? What is the difference between DOT 3, 4, 5, and 5.1? Mineral oil base versus silicon versus synthetic?

Let's start out with DOT 3, 4, and 5.1. The DOT stands for Department Of Transportation. This is a "big brother" that watches out for us and sets standards for such things that affect safety, such as brake fluid and tires. These fluids are made from what are called polyglycol ethers. Their drawbacks are that they have the ability to absorb water, a quality that is often referred to as "hydroscopic." Water, however, is not a good thing, as it causes corrosion and lowers the boiling point of the fluid. It doesn't take much moisture to lower the boiling point of brake fluid. DOT rates the boiling point on two different scales. One is called the dry and the other is the wet. "Wet" means the fluid has absorbed just three percent water by volume. This chart will give you an idea of just how they compare. As you can readily see, it doesn't take much moisture to cause a big effect.

Okay, but what is wrong with the boiling point being lowered? The big thing is that if it should boil, it causes little bubbles to form. Bubbles can be compressed. When they are compressed, it takes more pedal pressure and movement to apply the brakes-so much that the brake pedal may go all the way to the floor. Some people refer to this as "brake fade," but it actually should be called "fluid fade." Brake fade occurs when the friction material gets so hot, they start off gassing their binding agent. This gas forms between the drum or rotor, and the shoe or pad, and prevents full contact. Would you ever get the fluid that hot? Yes, you could on a long downhill grade, especially when towing or carrying a heavy load and you're using your brakes a lot, or when playing racer on the street or in the dirt.

How does moisture get into a sealed system? First off, every time you take the cap off the master cylinder to check the brake fluid, you're allowing moisture to get in. The same goes with the cap on the can. Can't believe that? Air usually has a moisture content (i.e., humidity) of anywhere from five to 99 percent, depending on where you live. Polyglycol-type fluids love to absorb this moisture. Ever spill some on your hands? Notice how it dries out the skin as it pulls the moisture from it? It's really important that you keep the lid on the master cylinder and the can, except when adding brake fluid to the system. Also keep in mind that while it is supposedly a sealed system, the seals are designed to keep the fluid in as pressure is put on them. When they relax, air-and the moisture in the air-can sneak past the seal.

Oh yeah, and speaking of air. As we learned in high school, while air can be compressed, a liquid cannot. However, in some cases it can be, strange as it sounds, especially in the case of polyglycol- and silicone-type fluids. However, the polyglycol types are two times less compressible than the silicone type fluids.

Let's go back to the DOT ratings for just a minute. Keep in mind that these DOT ratings are the minimum boiling points. In fact, some fluids in all categories have a much higher boiling temperature than those listed under the dry rating. Mainly used in racing, they may not have a higher wet boiling point. So they may or may not have a DOT rating.

Now, let's not forget the DOT 5 rating. These are silicone-based (sometimes referred to as "synthetic") brake fluids. Silicone-based fluid has several disadvantages and advantages. For starters, it has a higher-rated boil point both at wet and at dry. Now this wet boiling temp is something that I don't understand, as synthetic brake fluid will not absorb water, so why the lower boiling point? Must have something to do with the way it's tested. This means that you don't have the corrosion problem, and it doesn't react to paint and work as a stripper. Classic car and show-car owners love this stuff. DOT 5 does not mix well with polyglycol fluids, so you cannot mix them and must flush the system out completely if you plan to use them. I believe that the military uses synthetic DOT 5 fluid due to the fact that their vehicles often sit in storage for long periods of time.

Now for the bad or major disadvantage: For some reason, the stuff is compressible and will give you a somewhat spongy pedal. I tried it some years ago and didn't like the way the pedal felt. Race vehicles do not use synthetic fluid. If you somehow should get any moisture in the system, it will not be absorbed and will give you what's referred to as a "water/fluid/water/fluid situation." So as the water boils at 212 degrees and the brake fluid boils at, say, above 518 degrees, the water will form vapor bubbles of steam that can be compressed and will really cause a drop in pedal application. As they expand, these steam bubbles can also push against the system, causing the pads to push outward against the calipers, cause drag, and even lead to higher fluid temperatures.

Next month, we'll go into more about brake fluid, why you should periodically change it, and the proper way to bleed the braking system.