This is an excellent read...
http://www.thebrakeman.com/fluid_tech
One of the more interesting topics of discussion around the race tracks the past few years has been the controversy over the choice of brake fluid, specifically silicone or the more conventional polyglycol fluid, (known at the race tracks as DOT 3,4 & 5.) Since the discussion usually ends up being decided by opinion, and since opinions are worth exactly what you pay for them, I have decided to supply you with some interesting factual data about each to allow you to make up your own mind. (I'll also make it clear as to my preference and why.)
A hydraulic brake system must have brake fluid in order to function properly. The level at which the brake system does operate is dependent on the choice of fluid. If you chose to use anything else, for instance, water, the problems would become immediately apparent. To avoid problems, brake fluid must be used. Among other things, the more important elements of brake fluid are high boiling points, consistent viscosity and good lubricating ability. All brake fluids commonly used in automobiles sold in the United States, are regulated by the department of transportation (DOT). The container will have a number such as DOT 3, which refers to the DOT test designation that the fluid meets. Please note, the number designation does not imply a rating for comparison between the fluids. It only refers to the number of the test! (see table for minimum boiling points)
DOT MINIMUM BOILING POINTS
DRY BOILING POINT.........WET BOILING POINT
DOT 3......... 401 F ........... 284 F
DOT 4......... 446 F ........... 311 F
DOT 5..........500 F ........... 356 F
Both DOT 3 and DOT 4 fluids are normally polyglycol base fluids. Polyglycol fluids are hygroscopic, which means they absorb moisture. As the amount of moisture absorbed increases, the point at which the brake fluid boils, decreases. It just so happens that polyglycol based fluids are so hygroscopic, they will suck moisture right through the pores of a cast master cylinder. In a well sealed brake system, on the typical passenger car, these fluids will require changing approximately every 1-2 years, depending on the severity of use. In a race car, however, brake fluid may require changing as frequently as every single race.
A Winston Cup race car must begin every race with fresh fluid. If it does not, the boiling point over a 500 lap event could drop to the wet boiling point prior to the end of the race, due to the speed with which brake fluid can assimilate moisture. This can, and would, contribute to an inconsistent pedal height, and premature loss of brakes altogether.
Another problem which will cause more rapid destruction of the brake fluid is that racers have access to a variety of aftermarket master cylinders which do not have diaphragms that tightly seal the brake fluid from the air. Use of these types of master cylinders should be avoided, since the moisture in the air will be assimilated by the brake fluid immediately. This is why you should never buy brake fluid by the gallon, because once you have used the quart you needed to freshen your system, the moisture in the air inside the can will ruin the remaining fluid in the can. It is for this same reason you should never reuse fluid removed from your system. Once your system has been filled, re-place the cap immediately to avoid affecting the fresh fluid. We never package brake fluid in a container larger than 500 ml.
Never put any fluid in your brake system unless you are absolutely sure as to what it is. Petroleum products, such as oil and transmission fluid can quickly destroy the seals and cause major brake failure. Some import cars do not use fluids compatible with those used in this country. For instance, Rolls Royce uses aircraft mineral fluid in the brake system that would quickly destroy your brakes, so it is not even safe to assume that if it says brake fluid, its Ok. Always look for a DOT number. DOT 3 and 4 can be mixed, but DOT 5 should never be mixed with any other fluid. DOT 5.1 can be safely mixed with either 3 or 4. Both DOT 3 and DOT 4 fluids are corrosive and therefore, will attack paint. Since most race car owners pride themselves on the appearance of their cars, this would sound like the "kiss of death" for these brake fluids, but let's not jump to conclusions until we finish exploring all the facts. If brake fluid is spilled onto the paint, flushing with water and quickly wiping up the mess will not result in any damage. This should not be any problem for a racer as a bundle of clean rags is standard equipment.
DOT 5 fluid uses silicone as its base. DOT 5 is specifically, silicone based synthetic. Silicone has none of the problems we have described above because it is not hygroscopic so it does not absorb moisture. Also, it does not attack paint, so spillage is not a problem. Unfortunately, it does have some major drawbacks. First, it is extremely expensive often costing 5-10 times as much as polyglycol fluids. Secondly, and more importantly, the compressibility of silicone fluid is very unstable throughout the temperature range encountered in normal usage. As the temperature increases, the pedal travel necessary to compress the fluid changes, resulting in unpredictable pedal height.
Recently, a new synthetic fluid has been developed and is labeled under the description, DOT 5.1. This new fluid has improved characteristics for moisture absorbtion and compressibility. Now, the fluid can resist moisture contamination longer and provide superior performance for severe use. It is quickly becoming the fluid of choice for race cars. Let?s take a look at the individual factors, starting with compressibility.
Why is compressibility so important? Let's take an extreme example to explain this. If you took one cubic foot of air at atmospheric pressure, you could compress it into 1/2 cubic foot of space. The pressure would be higher, but the space it took would be smaller. You could then compress this 1/2 cubic foot of air into 1/4 cubic foot, and the pressure would be increased, but again, the volume would get smaller. If a brake fluid is truly a high quality fluid, the volume needs to be the same size, or nearly so, whether it is at rest, with no pressure, or under maximum racing pressures of 1500 PSI (Pounds per square inch). Not all DOT 3, 4 and now, 5.1 fluids are created equal. In fact, a good race car driver can often feel the difference in the consistency of the pedal height from cold to hot, as the temperature in the system changes over the course of the race.
Altitude also affects the compressibility of the fluid further compounding the problem of pedal travel. This would not be such a problem except for the fact that as we discussed in an earlier article, we must not only be able to develop pressure in a brake system, we must also be able to remove that pressure. In the case of silicone fluid, the expansiveness can be so severe that when you take your foot off the brake, residual pressure could remain high enough to cause brake drag. Consequently, that would cause further heating of the fluid and therefore further expansion, therefore more excessive drag that would end up locking the entire brake system, rendering the auto unable to be moved. For this reason, race teams that have experimented with silicone fluid long ago found they could not use it. When early NASCAR teams attempted to use silicone based fluids in the early 70's when disc brakes first became popular, it was not unusual to see a car stopped on the track with the brakes fully locked up. Once the system cooled down, the brakes released and the car could again move, but I can assure you many of the more senior drivers have seen this situation more than once.
Another characteristic of the fluid is if you have experienced excessive sponginess in your pedal after the car is used for a while, then you now know the most likely cause. What you may not know is that you may be tremendously shortening the life of your brake pads due to excessive dragging. The other factor that has come to light recently is silicone brake fluid affects the seals found in standard brake systems.
Ethylene propylene rubber seems to loose its durometer (hardness) when exposed to certain silicone fluids. Although not the case with other synthetics, it is very true in the case of silicone. This results in a change in the size and an increase in the wear rate of all rubber components in the system. Some silicone fluids affect rubber more than others. To further complicate the problem, if your system has the master cylinder mounted under the floorboard and you don't have a two pound residual valve (see last issue) present, you may have trouble identifying the problem due to the absence of this most important valve.
Street rodders are avid users of silicone brake fluid, primarily because of the expensive paints jobs they have on their cars. Unfortunately, since many of their brake systems are composed of components off of other vehicles, the additional problem of the wrong brake fluid makes many of these cars very unsafe.
Regardless of which fluid you use, here are a few quick tips on bleeding your brakes. First, you don't need a gallon of fluid to flush the system. Two twelve ounce cans will be adequate for most race cars. Secondly, and most importantly, do not pump up the pedal when first filling the system or at any time. When you pump the brakes you aerate the fluid creating thousands of little bubbles. This makes it even more difficult to get the air out of the system. Think of the master cylinder as a hypodermic needle and you are going to inject the brake fluid into the brake line. Push the pedal to the floor while another person opens the bleed screw. After the bleed screw has been closed, lift your foot off the pedal, which will allow the piston to retract which reloads the bore with fluid from the reservoir. Repeat the process twice on each bleed screw around the entire car. This will fill the lines and pistons, removing the majority of air. Now you are ready for final bleeding. You will have resistance present when you push down, although the travel may still be more than desirable due to the presence of additional air. depress the pedal until resistance is felt, usually about 20-30 pounds of foot pressure. Have the other person open the bleed screw and observe what comes out. If you are pushing solid fluid, the air is most likely gone from the portion of the system controlled by that bleed screw. Move to another bleed screw until no more air is found.
Racers should always use a high quality fluid designed for racing. Experiment with different brands, there is a definite difference in how the pedal will react. Be careful of inexpensive fluids, although there are some brands that are good, many lower priced brake fluids are poor and will result in premature brake failure. If you have had a problem with a spongy pedal late in a race, this is the first item you should change.
In essence, the point of this entire article is that although both types of fluids have advantages and disadvantages, the bottom line is that the disadvantages of silicone brake fluid render it unacceptable as a safe and reliable alternative to polyglycol fluids under all conditions. In other words, in my opinion, it has no place in any race car! On the other hand, although DOT 3, 4 and 5.1 fluids have problems, these problems are controllable given reasonable care and maintenance. We all have a tendency to take the pedal travel of our brake pedal for granted, but I want to assure you that there is no fear any worse than the fear of stepping on the pedal and finding nothing there. Consistent even pedal travel is extremely important to safe stopping distances. It is even more important to giving a race car a consistent, repeatable feel to the pedal, so the driver knows he can depend on maximum performance.
Whether you own a race car or a street rod, or in fact, any car, this article offers some critical information for safety's sake. The next time you see a bottle of brake fluid on sale for $2.97, you may want to ask yourself if what you are buying truly will handle your intended use. What may be OK to stop you passenger car at 55mph, probably has no place in a race car at 125mph.
Everything You Need To Know About Brake Fluid
Linear Mode
