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 by opinion, and since opinions are worth exactly what you say 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 will not operate without brake fluid. To a large degree tthe level at which the brake system does operate is dependant on the choice of fluid. 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 can will have a number such as DOT 3, which refers to the DOT test designation that the fluid meets. Please note, the number disgnation does not imply a rating for comparison between the fluids. (Refer to table for minimum boiling points.)
DOT Minimum Boiling Points | ||
|---|---|---|
| Type | Dry | Wet |
| DOT 3 | 401 F | 284 F |
| DOT 4 | 446 F | 311 F |
| DOT 5 | 500 F | 356 F |
Polyglycol fluids are hygroscopic, which means they absorb moisture. As the amount of mooisture absorbed increases, the point at which the brake fluid boils, decreases. In a well sealed brake system these fluids will require changing approximately every 1-2 years, depending on the severity of use. Unfortunately, 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 fresh fluid.
Never get 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. Both DOT 3 and DOT 4 fluids 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 fro a race as a bundle of clean rags is standard equipment.
DOT 5 fluid uses silicone as its base. 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 no 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 compressiblity 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.
Altitude also affects the compressiblity of the fluid further
compunding 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 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.
It is for this reason race teams that have experimented with silicone fluid
long ago found they could not use it.
Another characteristic of the fluid is if you experienced excessive sponginess in your pedal after the car is used for a while, then you know the most likely cause. What you may not know if that you may be tremendously shortenin 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 silicone fluid. This results in a change in the size and an increase in the wear rate of all rubber components in the system. 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,
primaril because of the expensive paint 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.
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. On
the other hand, although DOT 3 and 4 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.
Whther 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 $2.90 you may want to ask yourself is what you are buying truly will handle your intended use. What may be OK to stop your passenger car at 55mph, probably has no place in a race car at 125mph.
Editors Note:
Warren Gilliland, President of "The Brake Man!", has over 27 years in the
design ad development of brake systems for industrial and racing
applications. He feels that the need for technical support on all products
used on race cars is essential and supplies in-depth instruction with all of
the products he supplies. This article was supplied courtesy of "The Brake
Man!" to assist you in your racing activities, and make your vehicle safer
and more reliable.
>From TurboTim
Comparison of high performance brake fluids on the market.
Dry Boiling
Point
Petrosin 500 Castrol LMA Not sure ? 500 Motul 5.1 509 Motul 300oC 572 Ferodo 554 Girling Dot 5.1 552 Brake Man: 570 H.P. Fluid 570 ATE Super Blue 536 ATE Dot 4 509
Jan you may want to add this to a FAQ.
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