All About Brake Fluids
This is long but I hope you find it informitive.
All About Brake Fluids
On Internet sites or even on the shelf of your neighborhood auto parts store you will find that good old Dot 3 brake fluid shares the shelf with an array of different fluids all with their own claim to fame. This temps many to replace their brake fluid with something other then the conventional DOT 3 fluid that is recommended by the manufacture. Many think they are making an improvement by doing so. But few understand what the real differences are in brake fluids and the fact that some fluids are not intended for use in an average daily driver. Some fluids, if not properly install can cause severe damage to the brake system or even brake loss.
In my many years as a brake technician I have seen my share of damaged brake systems because the wrong fluid was used or used improperly. It has always struck my interest in the differences in brake fluids and their use. Therefore I would like to pass on some of this knowledge so you can make an informed decision on whether to change to a different fluid or stay with good old DOT 3.
Brake Fluids on The Shelf
DOT 3
This is the type used in most production vehicles. This fluid is hydroscopic (meaning it will attract and suspend moisture. It must have a “dry” (contains no water) boiling point of no less then 401° F and a “wet” (saturated with water) boiling point of no less then 284° F. It must also meet certain stability and corrosion resistance standard as set by HTSA guidelines for DOT 3 brake fluids. This fluid must be clear to amber in color.
DOT 4
This fluid can be found in many European vehicles but can be used in others. You should always check the fill cap or owners manual to verify if DOT 3 or DOT 4 is required. This fluid is also hydroscopic. It must have a dry boiling point of no less then 446° F and a wet boiling point of no less then 311° F. It must also meet certain stability and corrosion resistance standard as set by HTSA guidelines for DOT 4 brake fluids. This fluid must be clear to amber in color.
DOT 4 fluid can be used in place of DOT 3 to raise the boiling point. The two are compatible and can be mixed. But never used DOT 3 when DOT 4 is required by the manufacture.
DOT 5
This is the other animal! DOT 5 is Silicon based fluid and is not hydroscopic. It must have a dry boiling point of no less then 500° F and a wet boiling point of no less then 356° F. It must also meet certain stability and corrosion resistance standard as set by HTSA guidelines for DOT 5 brake fluids.
This fluid is not compatible with DOT 3 or DOT 4 fluids, for this reason it is usually a purple color so it will not be confused with conventional DOT 3 or 4 fluids. It cannot be mixed with other grades of brake fluids and cannot be simply “flushed” into a system. It cannot be used with ABS brakes systems and cannot be used in PowermasterÒ brake systems. I will explain why later.
DOT 5.1
This is where reading the label is a must. DOT 5.1 as considered by the HTSA is a non-silicon based fluid with the same minimum dry and wet boiling point as DOT 5 or higher. Many will meet or accede a dry boiling point of 585° F.
Why is reading the label a must? These fluids are on the line between convention fluids and Racing Fluids and may not comply with all of the stability and corrosion resistance standard as set by HTSA guidelines. Check with the manufacture for compatibility before using, but these fluids are usually considered the best for street and heavy track use.
Racing Fluids
Racing fluids are really not covered under any HTSA standards though they can be DOT compatible as far as boiling points and rubber part compatibility. They can rate a dry boiling point range of 446° F to 620° F depending on the brand. Most are hydroscopic unless silicon based.
The problem with these fluids are they do not meet the same stability and corrosion resistance standard as other DOT rated fluids. This is fine for racing as the fluid is usually changed between races or very frequently. This leaves little time for these fluids to break down. These fluids should never be used in street vehicles that will not have frequent fluid changes.
Synthetic
Synthetic fluid, not to be confused with silicon are relatively new to the market and use the same basic polyethylene glycol base as conventional fluid (meaning simply they are hydroscopic); the difference is that this base has been "synthesized" by the manufacturer to improve it. Synthetic fluids are completely compatible with conventional DOT 3-4 fluids and will not harm seals or rubber parts. Most have a wet and dry boiling point that meets or accessed the DOT 4 standard. Synthetic fluid can also be found in DOT 5.1 forum also.
Synthetic fluids will usually accede the HTSA standards for stability and corrosion resistance depending on the manufacture. These fluids can be clear to amber in color showing their compatibility with convention brake fluids.
Other facts about conventional brake fluid
One thing that strikes people is the term “hydroscopic” this is the ability of a substance to attract water molecules from the surrounding enviroment through absortion. Right away people believe this is a bad thing. But brake fluid is desighned this way for reasons. The hydroscopic properties also allow brake fluid to suspened or hold any water molecules that do get in the fluid. Moisture then cannot collect or puddle in any one area and cause isolated corossion or a “hot spot” that would allow a boil, dropping the fluids rated boiling point drastically. For this reason hydroscoic brake fluid is actually a good thing.
You may then ask, “isn’t this the reason I must chanage my brake fluid every few years?” Well, not exactly. It has been a common misconseption that moisture laden brake fluid is the main cause of intrena corrosion and parts damage on neglected systems. The fact is that brake systems are sealed and neglected ones are seldeom opened. Moister can be drawn in through rubber parts and hose, but this may only be by about 1% per year possibly less with vast inprovement in rubber parts.
What has been found is that the corrosion inhibitors in the fluid tend to break down way before the systems become moisture saturated. Cooper ions released from metal brake lines can travel through the system interacting with other metals and causing the corrosion process to begin.
Reccently new guidline have been adopted by the Motorist Assurance Program (MAP) this group sets uniform standards for automotive repairs called Uniform Inspection and Communication Standards.
MAP standards for brake fluid:
Suggest; testing brake fluid at OE-recommended brake system inspection service intervals to insure copper content is below 200 ppm.
Require; brake fluid replacement if copper content exceeds 200 ppm.
Suggest; brake fluid replacement at vehicle-specific OE replacement intervals (if they exist).
Test strips are available for testing copper content and this is what should be used to tell if brake fluid is “bad” and requires a change.
Then again if you follow the manufactures recommendation or change your fluid at least any time brake service is done, you should have no problems. Change 2-3 years is a safer bet.
Silicon DOT 5 brake fluid and how to use it
There is really no reason to use this fluid in anything other then a museum piece or a vehicle with extraordinary custom paint work that may be damaged from conventional brake fluid exposure.
This fluid should absolutely never be used in a vehicle with ABS, VSC or when brake fluid is used as to operate the power brakes as in a GM PowermasterÒ system. One problem with DOT 5 is it aerates easily, and holds air between the molecules. The above systems tend to “whip” up the fluid and this will cause silicon fluid to foam and can resulted in loss of pedal pressure.
Silicon fluid cannot be mixed in any way with conventional fluid or can it be “flushed” into a system that has used conventional fluids. If this is done, damage to the system will eventually occur.
The reason why this will not work goes back to conventional brake fluids hydroscopic properties. Conventional DOT 3-4 Brake fluid will permeated rubber parts and will continue to attract moisture. Consider that one drop of conventional brake fluid is just as hygroscopic as a gallon. If any moisture enters the system this attraction can cause failure in very specific area where the conventional fluids can remain. To use silicone fluid all rubber must be replaced and all traces of old fluid flushed from the steel lines and other components.
Other Fluid Use
Synthetics are the new kid on the block and are the only real substitute for conventional brake fluid to consider. They need special mention here as many mix theses up with silicon fluids. They are not the same!
Most vehicle, unless required by the manufacture do not need anything other then conventional DOT 3 brake fluid. Since most synthetics meet or exceed DOT 4 requirements, and have improved corrosion resistance properties, this makes them a great upgrade to conventional fluids.
Keep in mind that synthetic brake fluid like with synthetic oils do not have an extended service life. These fluids should still be changed at least at every brake service. So you must consider if the extra cost is worth it to you.
On the other hand if you do tend to push your fluid changes to every brake job and not every other year synthetics can be a better choice.
Here is a quick out line of how brake fluids should be used
1) DOT 3 conventional brake fluid, used in most vehicles from the factory check cap or owners manual Color clear to amber.
2) DOT4 conventional brake fluid, used in may European vehicles check cap or owners manual. Can be used with DOT 3 system to raise boiling point. Color clear to amber.
3) DOT 5 Silicon brake fluid, use only in vehicles that are stored long term, or all out custom show vehicles. Does not mix with conventional brake fluid systems, cannot be “flushed” in. Cannot be used with ABS, VSC or PowermasterÒ systems. Color violet or purple.
4) DOT 5.1-brake fluid, used when a higher boiling point then conventional fluid is needed as in high performance street and track used vehicles. Check manufactured DOT specifications for proper corrosion protection and long-term use. Color clear to amber.
5) Racing brake fluids, use only on race vehicles. Not to be used on street vehicles. Fluids are expensive and much be changed on a regular basis. Most lack any long-term corrosion protection and do not meet DOT specification in these regards. Color, can be any.
6) Synthetic brake fluid, can be used in place of conventional DOT 3or 4 fluid. Should never be mixed with silicon fluid. Has improved boiling points and corrosion protection. Color clear to amber.
A Word On Contaminated Systems
I have seen many contaminated systems over the years and most are not a pretty sight. According to MAP guidelines a system can be contaminated in two ways. A system can be contaminated if a fluid other then brake fluid is used that will not attack rubber seals and hoses. About the only fluids that would fall in this category are water and the wrong grade of brake fluid, such as DOT 3 in a DOT 4 system or silicon fluid in an ABS system. In this case simply flushing out the system with the proper fluid is all that is required.
If water or silicon fluid has been added, time is not on your side. Water will dissipate and lower the boiling point but large quantities can start causing damage to metal parts. Silicon fluid if flushed in without changing rubber parts can cause similar damage if left in the system as has been described.
Now if a system is contaminated with a fluid that is not compatible with seals and rubber parts, this is a major problem. Most times this is a mineral based fluid, the two most common being power steering fluid or automatic transmission fluid. In this case a complete system overhaul will be required and all rubber parts must be replaced, including the ABS unit if equipped.
If you suspect a mineral base fluid has been added, the first sign being swelling of the master cylinder cap rubber, you can sometimes tell what has been added by pulling out some fluid from the master cylinder and adding it to a clear cup of water. Brake fluid will dissipate in the water but the oils will separate and float to the top of the glass where it can be examined.
This information is true to the best of my knowledge. Something can change as new information becomes available. Source for this information came from NHSA DOT guidelines for automotive brake fluid, Andrew Markel editor of Brake and Front End magazine, MAP Guidelines on brake fluid service and replacement and various other magazine articles.
Mike
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“The significant problems we have cannot be solved at the same level of thinking with which we created them” (Albert Einstein)
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