Here's a little essay on silicone brake fluid. I would give credit to the author, but all the places this has been posted lack any mention of the author so I have no one to credit this to.
Also, as Jeep driver, silicone brake fluid is a big no-no at high altitudes. It tends to foam quite a bit coming through a small orofice when up in the mountains (4000 feet and higher).
Due an internet search on the subject for all the info, but here is a little writeup to start out with:
"Automotive braking is, as we all know, a really important factor in performance car building. The forward thinkers pay an equal amount of attention to stopping as they do to making their vehicles go fast. But, here's a fact that not too many realize: when you step on the brakes, you command a stopping force that's 10 times as powerful as the force that puts the car in motion. Hard to believe isn't it? No wonder the performance brake market has been vying for and finally receiving our attention. But now that they've educated us in the importance of quality components, it's now time to pay attention to the fluid that activates and allows these awesome components to work.
It only makes sense that after you've spent big bucks and a lot of time upgrading your vehicle, the completion of your job must go beyond adding just any old fluid into the system. It's extremely important to always use a high quality fluid. Let's face it, that quart of Elmo's Wonder Brake Fluid and Windshield Washer Solvent combo you picked up at the swap meet may end up as the only thing between the ball of your foot and a brick wall. So, let's at least pay a fraction of the attention spent on choosing components on choosing brake fluid. The following information should help you better understand the differences between various types of fluids.
Brake fluid must function through heat, cold, and all the variations of temperatures affecting all brake systems. In other words, brake fluid must be and must remain viscous (fluid) and resistant to severe conditions. It must also absorb water without having an affinity for doing so. Brake fluid must be pure and contain no contaminants, which will have a harmful effect on rubber components. It also should serve as a lubricant for the moving parts in a brake system and be able to withstand time and service--in other words, its boiling point and chemical properties must remain stable. One more important factor is that brake fluid should cause an electrolytic action that will decompose or degrade metal brake parts.
These factors are taken into consideration by the Department of Transportation, National Highway Safety Administration in Federal Motor Vehicle Safety Standard 116. FMVSS116-classified fluids fall into three categories: DOT 3, DOT 4, and DOT 5. DOT 3 meets the minimum specifications, while DOT 4 and 5 meet more stringent requirements. The purpose of these requirements is "to reduce failures in the hydraulic brake systems of motor vehicles which may occur because of the manufacture or use of improper or contaminated brake fluid." In addition to setting the minimum wet and dry boiling points for all brake fluids, Federal Standard 116 deals with fluid viscosity, temperature and chemical stability, corrosiveness, water tolerance, compatibility (between formulations), and effects on brake system components.
OK, WHAT'S THE DIFFERENCE?
It seems logical that if DOT 3 is good enough then DOT 4 must be better. And, if DOT 4 is better, then DOT 5 must be better still. OK then, why? It all boils down to boiling point. When brake fluid begins to boil (from the heat that's generated in the system during braking) it forms gas bubbles. As these bubbles collect and grow, they form pockets of gas in the system. And since gas is much easier to compress than fluid, these pockets cause the pedal to easily compress or go soft. This is what as known as a spongy pedal--a condition that causes the heart to race.
The following chart shows the aforementioned FMVSS116 minimum boiling points. These boiling points are the biggest difference in the fluid classifications. You'll notice there are two points for each classification of fluid. The dry boiling point is for fresh fluid that has not yet absorbed moisture. The wet boiling point is for fluid that has been in use (or in an opened container) long enough for it to absorb moisture from the atmosphere.
CONNECTING THE "DOT"(S)
The different DOT spec fluids have chemical or formulation differences, as well as boiling point differences. There are (at this point) three commercial formulations for brake fluids; two are glycol-based and the third is silicone-based. DOT 3 brake fluids are a mixture of polyalkylene glycol ether and other glycols (all start life as ethylene glycol--antifreeze); DOT 4 fluids add borate esters to raise the boiling point. DOT 5 is silicone oil based with additives.
SILICONE BASE BRAKE FLUID (SBBF)
The U.S. DOT defines silicone brake fluid as that which consists of no less than 70 percent of adiorgano polysiloxane by weight. Silicone-based fluids are regarded as DOT 5 fluids. They are highly compressible and can give the driver the feeling of a spongy pedal. The higher the brake system temperature, the more the compressibility of the fluid--increasing the feeling of a spongy pedal. Silicone-based fluids are non-hydroscopic, meaning that they will not absorb or mix with water. When water is present in the brake system, it will create a water/fluid/water/fluid situation. Because water boils at approximately 212 degrees F, the ability of the brake system to operate correctly decreases, and the steam created from boiling water adds air to the system. It is important to remember that water may be present in any brake system. Therefore, silicone brake fluid lacks the ability to deal with moisture and will dramatically decrease a brake systems performance. Silicone brake fluid has a number of strengths and drawbacks.
1) It has a high boiling point since it does not absorb water. Therefore, there's no so-called wet boiling point.
2) Doesn't absorb moisture.
3) Doesn't remove paint.
4) The viscosity is more stable over the extremes of temperature.
5) With the exception of some formulations used in external boots, silicone brake fluid is compatible with all standard brake components.
Drawbacks:1) It's hard to pour without entraining air bubbles--hence an application will generally have a softer, spongier pedal feel.
2) It doesn't absorb water, so any water already in the system accu-mulates in the lowest point of the system and stays there, causing rust.
3) Glycol fluids begin to compress near their boiling points, whereas silicone fluids begin to compress at around 300-350 degrees Fahrenheit.
4) Additives in the fluid can vaporize at comparatively moderate temperature, increasing the spongy feel.
5) Silicone fluids expand significantly when hot.
6) Silicone fluid is functionally incompatible with systems that have held glycol-based fluids for any length of time, requiring flushing and seal replacement (there are counter opinions on this, which state that the modern silicone formulations are in fact compatible with only a flushing, rather than a complete reseal). The actual DOT specification requires chemical compatibility, so as far as that goes, the two fluids won't cause reactions if used in the same system, but they certainly won't mix, either.
7) It's pretty much incompatible with anti-lock brakes because the silicone fluids tend to be more viscous, which can cause problems with the timing of the pulses that are intended to work with the thinner glycol-base fluid. This sometimes leads to damage of the ABS valving. The rapid pulsing necessary to anti-lock functions tend to cavitate the fluid, as the tiny bubbles collapse and coalesce into larger ones, and then collapse and reform into smaller ones. This tends to counteract the ABS effect and can diminish the actual effective braking. This condition also heats the fluid and can lead to even more sponginess and possible damage to the ABS controller. Thirdly, silicone brake fluid tends to foam when expressed from a small orifice under pressure, reducing its hydraulic effectiveness greatly.
Minimum Dry Boiling Point Minimum Wet Boiling Point
(All degrees in Fahrenheit)
DOT 3 401 28
DOT 4 446 311
DOT 5 500 356"