Brake Fluid Facts 

Description Brake Fluid Facts
Author Matthew Date Fri Apr 25, 2008 3:49 am Type Type 1
Category Tech Tips
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Brake Fluid Facts
by Steve Wall

As a former materials engineering supervisor at a major automotive brake system supplier, I feel both qualified and obligated to inject some material science facts into the murky debate about DOT 5 verses DOT 3-4 brake fluids. The important technical issues governing the use of a particular specification brake fluid are as follows:

Fluid compatibility with the brake system rubber, plastic and metal components.
Water absorption and corrosion.
Fluid boiling point and other physical characteristics.
Brake system contamination and sludging.
Additionally, some technical comments will be made about the new brake fluid formulations appearing on the scene.

First of all, it's important to understand the chemical nature of brake fluid. DOT 3 brake fluids are mixtures of glycols and glycol ethers. DOT 4 contains borate esters in addition to what is contained in DOT 3. These brake fluids are somewhat similar to automotive anti-freeze (ethylene glycol) and are not, as Dr. Curve implies, a petroleum fluid. DOT 5 is silicone chemistry.

Fluid Compatibility
Brake system materials must be compatible with the system fluid. Compatibility is determined by chemistry, and no amount of advertising, wishful thinking or rationalizing can change the science of chemical compatibility. Both DOT 3-4 and DOT 5 fluids are compatible with most brake system materials except in the case some silicone rubber external components such as caliper piston boots, which are attacked by silicon fluids and greases.

Water absorption and corrosion
The big bugaboo with DOT 3-4 fluids always cited by silicone fluid advocates is water absorption. DOT 3-4 glycol based fluids, just like ethylene glycol antifreezes, are readily miscible with water. Long term brake system water content tends to reach a maximum of about 3%, which is readily handled by the corrosion inhibitors in the brake fluid formulation. Since the inhibitors are gradually depleted as they do their job, glycol brake fluid, just like anti-freeze, needs to be changed periodically. Follow BMW's recommendations. DOT 5 fluids, not being water miscible, must rely on the silicone (with some corrosion inhibitors) as a barrier film to control corrosion. Water is not absorbed by silicone as in the case of DOT 3-4 fluids, and will remain as a separate globule sinking to the lowest point in the brake system, since it is more dense.

Fluid boiling point
DOT 4 glycol based fluid has a higher boiling point (446F) than DOT 3 (401F), and both fluids will exhibit a reduced boiling point as water content increases. DOT 5 in its pure state offers a higher boiling point (500F) however if water got into the system, and a big globule found its way into a caliper, the water would start to boil at 212F causing a vapor lock condition [possible brake failure -ed.]. By contrast, DOT 3 fluid with 3% water content would still exhibit a boiling point of 300F. Silicone fluids also exhibit a 3 times greater propensity to dissolve air and other gasses which can lead to a "spongy pedal" and reduced braking at high altitudes.

DOT 3 and DOT 4 fluids are mutually compatible, the major disadvantage of such a mix being a lowered boiling point. In an emergency, it'll do. Silicone fluid will not mix, but will float on top. From a lubricity standpoint, neither fluids are outstanding, though silicones will exhibit a more stable viscosity index in extreme temperatures, which is why the US Army likes silicone fluids. Since few of us ride at temperatures very much below freezing, let alone at 40 below zero, silicone's low temperature advantage won't be apparent. Neither fluids will reduce stopping distances.

With the advent of ABS systems, the limitations of existing brake fluids have been recognized and the brake fluid manufacturers have been working on formulations with enhanced properties. However, the chosen direction has not been silicone. The only major user of silicone is the US Army. It has recently asked the SAE about a procedure for converting from silicon back to DOT 3-4. If they ever decide to switch, silicone brake fluid will go the way of leaded gas.

Brake system contamination
The single most common brake system failure caused by a contaminant is swelling of the rubber components (piston seals etc.) due to the introduction of petroleum based products (motor oil, power steering fluid, mineral oil etc.) A small amount is enough to do major damage. Flushing with mineral spirits is enough to cause a complete system failure in a short time. I suspect this is what has happened when some BMW owners changed to DOT 5 (and then assumed that silicone caused the problem). Flushing with alcohol also causes problems. BMW brake systems should be flushed only with DOT 3 or 4.

If silicone is introduced into an older brake system, the silicone will latch unto the sludge generated by gradual component deterioration and create a gelatin like goop which will attract more crud and eventually plug up metering orifices or cause pistons to stick. If you have already changed to DOT 5, don't compound your initial mistake and change back. Silicone is very tenacious stuff and you will never get it all out of your system. Just change the fluid regularly. For those who race using silicone fluid, I recommend that you crack the bleed screws before each racing session to insure that there is no water in the calipers.

New developments
Since DOT 4 fluids were developed, it was recognized that borate ester based fluids offered the potential for boiling points beyond the 446F requirement, thus came the Super DOT 4 fluids - some covered by the DOT 5.1 designation - which exhibit a minimum dry boiling point of 500F (same as silicone, but different chemistry).

Additionally, a new fluid type based on silicon ester chemistry (not the same as silicon) has been developed that exhibits a minimum dry boiling point of 590F. It is miscible with DOT 3-4 fluids but has yet to see commercial usage.


Brake Fluids: DOT 4, DOT 5.1, and DOT 5 Silicone
Q: Sir Tech,
My bike's brakes are currently filled with DOT 4. Is it true that I will get better performance if I switch to Bel-Ray Silicone DOT 5 Brake Fluid?

A: No, Years ago, many road race teams used DOT 5 silicone brake fluid to get higher boiling points than the DOT 3 & 4 brake fluids of that era. DOT 5 silicone is not hygroscopic, meaning it won't absorb water from the air, so it retains its high boiling point over time. On the other hand DOT 3, 4 and 5.1 brake fluids are glycol based and do absorb water from the air, which then lowers the boiling point. That is why you will see wet and dry boiling points, with the dry boiling points always being higher. Glycol based brake fluids may start out with a fairly high dry boiling point but as they absorb moisture the boiling point decreases. So every time your brakes get really hot and then cool, condensation occurs and moisture is absorbed. That is why it is important to replace your brake fluid regularly. It may seem obvious that DOT 5 silicone is the best choice, but not so. Today most road race teams use DOT 4 or DOT 5.1 glycol based brake fluids because the dry and wet boiling points today are very high and in some cases higher than DOT 5 silicone. The number one reason not to use DOT 5 silicone is a slightly mushy lever or pedal feeling as compared to glycol based brake fluids. This is the nature of the product. Street riders may not feel the difference, but the race teams prefer the precise feeling of glycol. The only motorcycles that I know of rolling off the production line with DOT 5 silicone are harley-Davidson® and other American made motorcycles; and that is mainly where we sell our Bel-Ray Silicone DOT 5 Brake Fluid. I recommend you stay with the DOT 4 fluid for performance, not to mention the enormous job it is to switch your system over from glycol to silicone. If that job is not done properly, you run the risk of coagulation if remnants of DOT 4 are mixed with DOT 5 silicone. I hope that's not too much for you at once. Maybe you should read half, take a play break, then read the rest.

DOT 5 Silicone versus DOT 5.1

Q: Sir Tech,
The master cylinder cover of my bike says fill with DOT 5 brake fluid. Can I use your Bel-Ray Silicone DOT 5 Brake Fluid?

A: I'll bet your bike is of European descent! In that case do not use DOT 5 silicone in your brakes. Use DOT 5.1 brake fluid. There is much confusion around this dilemma and making an error could be serious. DOT 5 silicone brake fluid is generally not recognized in Europe, so they call DOT 5.1 DOT 5 for short. In America, the brake fluids' names are DOT 3, DOT 4, DOT 5 and DOT 5.1. All are glycol and compatible with each other except for DOT 5 , which is, obviously, silicone based. In Europe beware, they do not recognize DOT 5 silicone and, therefore, call DOT 5.1, which is a glycol product, DOT 5. This can cause great confusion and obvious problems. DOT 5.1 got its start during the introduction of anti-lock or ABS brakes, where a light viscosity brake fluid was used to assist the modulation of the anti-lock mechanism. Suddenly DOT 5.1 is in vogue today and many are buying it under the guise that it must be better than DOT 4 because it is the next sequential number. Not so! To get the best performance brake fluid, check the wet and dry boiling points. In many cases, depending on the manufacturer, DOT 4 has higher boiling points than DOT 5.1. Today, DOT 5 silicone brake fluids are mainly used in the harley-Davidson® motorcycles since this is what they come with from the factory. The advantage is that if any fluid spills, it won't damage the custom paint jobs. Do your community a favor and pass this info on to another European bike owner or, better yet, quiz your dealer to see if he is "in-the-know".

DOT 4 VS. DOT 5 Silicone Coagulation

Q: Sir Tech,
I bought a 1987 harley-DavidsonĘ Sportster and I recently removed the front brake master cylinder cover to find a bunch of thick gunk inside. What happened and what should I do?

A: If you own a harley-Davidson®, think purple! Your H-DĘ motorcycle came stock from the factory with DOT 5 silicone brake fluid and part of the regulated requirements for DOT 5 silicone is that it be purple colored so as to stand apart from the ever so popular DOT 3, 4 or 5.1 brake fluids which are glycol not silicone. Apparently, your hog's previous owner was more concerned with his tattoos turning green than with his brake fluid being purple and most likely added some DOT 3 or DOT 4 brake fluid to the system. This is a big no-no, bad boy! What happens when you introduce DOT 3, DOT 4 or DOT 5.1, all glycol based fluids, to DOT 5 silicone, which is obviously silicone based, is coagulation! The fluid will clot and, as you so eloquently described, form thick gunk. My advice to you is to consult a qualified technician. After all these are your brakes we are talking about! Have him fully flush the brake system and replace all the rubber parts with new ones and then go back to DOT 5 silicone brake fluid. If you do not re-build the master cylinders and calipers or replace all the rubber parts, there will most likely be remnants of the glycol in the rubber which will cause the problem again. To avoid "thick gunk" in your shorts due to grabbing the brakes and not impeding forward velocity, I recommend you heed my advice with the utmost seriousness and expedience.

User comments 
Matthew Posted: Fri Apr 25, 2008 3:49 am    Post subject [ KB ]

[ Article updated...Matthew ]

Category: Tech Tips
Type: Type 1

Article Name: Brake Fluid Facts
Author: Matthew
Description: Brake Fluid Facts

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