( This file is a combined version of 2 -3 files, recently held on this site...) Pointers in using silicone brake fluid... 1> NEVER allow the can of fluid to be shaken! Any bubbles introduced into the fluid must be allowed to settle out before using. Heating the fluid can speed this up and may take over 48 hours to clear out. Silicone fluid can contain microscopic bubbles that will give you a softer pedal, even after thoroughly bleeding the system. 2> In-line brake light switches are known to fail when using silicone fluid. The silicone attacks the switch in 2 ways. The oil molecules are so small, that it gets by internal seals and/or the silicone attacks the rubber in the diaphragm allowing fluid to pass and short the switch. The remedy for this is to install a new switch and use an eyedropper to fill the switch with regular DOT 3 or 4 before installing it. The fluid will never be flushed out and will protect the diaphragm from being attacked by the silicone.i An alternative is to install a mechanical brake light switch on the pedal itseld. A GM type is reported to be an easy adapt. 3> When installing a new Master cylinder, always carefully, bench bleed the unit before putting it on the car. This entails fabricating a means to connect a hose to each outlet port and pump the fluid back into the reservoir to charge the internal cylinder and orifices. Make the pumping action slow and deliberate or you get bubbles. 4> Silicone fluid is like synthetic oil. It can find its way out of any containment if there is the slightest space. On all fittings, use a liquid teflon sealant after thoroughly cleaning the threads with alcohol. Use a Q-tip to clean internal threads on the MC. 5> Silicone fluid will not absorb water, so if you are changing TO silicone, flush your system thoroughly with ALCOHOL, as this will absorb water. If any water is allowed to remain in the lines or calipers or cylinders, you can have some trouble, IE; The water will settle to the bottom of the system, as it is not imiscible (soluble). If the brake fluid gets over the boiling point, the water will vaporize and expand, locking your brakes. Cooling everything down is the only means out of this situation, but not with a hose. 6> In any event or pertaining to any type of fluid, it it recommended that you flush you system once every 2 years to assure trouble free operation. Using a Vacuum type 'one-man" bleeder at the RR wheel first, loosen the bleeder screw and draw out fluid. Check the reservoir every so often to allow topping off of new fluid before any air enters the piston chambers of the Master Cylinder. When the RR runs clear, move the the LR, the RF and last the LF. --------------------- Jim, Silicone brake fluid (DOT 5) is of great value, in my opinion, for cars not driven very much, as too many people tend to view periodic maintenance as something done on a mileage interval, rather than time interval, basis. I use it in three of my vehicles, two Studebakers and one motorcycle, and don't use it in two others, another Studebaker and our Saturn. DOT 5 has charhacteristics about it that must be understood and dealt with, or you'll regret having installed it. The most conservative view: 1. ALL, and I mean ALL, old DOT 3 or 4 fluid must be removed from your present brake system. This means disassembling all parts, including master and slave cylinders, to thoroughly clean them of old brake fluid with alcohol. DOT 5 does not mix with any other brake fluid. 2. DOT 5 is composed of smaller molecules, meaning it leaks past joints in brake line that DOT 3 or DOT 4 won't. You find yourself tightening everything, and replacing some junctions, in order to keep the DOT 5 inside your system. 3. While the rubber parts used in today's brakes does not react to DOT 5, the same cannot be said of the rubber bits used in the systems of older cars. Some stuff as current as the 1980's will swell up when exposed to DOT 5. Check with others about your current car and, if you switch to DOT 5, avoid NOS brake parts that the fluid will come into contact with. You'll find out soon enough if a part will work with DOT 5 after installing it on your car. 4. DOT 5 has a propensity to hold air in suspension within itself, making it very difficult to bleed. Never shake the can, always pour it out VERY SLOWLY and expect to spend extra time bleeding your brakes to expel all of the air in the lines. Can't think of anything else. Once you've switched, you can effectively kiss corrosion goodbye, as long as ALL of the old fluid is gone and you never mix DOT 3 or 4 with the DOT 5. Chris Altenburg ------------------------------------------------ Submitted to the Avanti group by a BMW engineer >>>>>> 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. <<<<<<<<< 1. Will not damage paint Dot 3 brake fluid will. 2. Perception that since Silicone will not absorb water it is less corrosive to calipers and wheel cylinders. 3. Will last longer in cars used only occasionally. 4. Higher boiling point. Most web sites I read recommended agaisnt Silicone fluid in brake systems built for Dot 3 type fluids. Here's why: 1. Silicone may damage seals in system compatible with Dot 3. 2. Silicone will give you a mushy pedal as is is more compressible than Dot 3. 3. Although water is immiscible with Silicone any moisture in system will sink to the lowest point and water will be suspended as droplets within the silicone and can still cause corrosion or since water is in droplets then the water may boil and resulting steam may lock brakes periodically. 4. Silicone must not be mixed with Dot 3 so if used system must be flushed with alcohol and dryed out with air. 5. If used frequent bleeding is recommended as air will also form trapped bubbles in system. Just pouring fluid too fast or shaking up can of fluid can cause trapped air. After considering all things and bleeding my old system and getting clean fluid with no rust or milky appearance, I am going back with a typical brake fluid (glycol base) with one twist. I contacted Castrol by e-mail and also got confirmation that Dot 3&4 fluid is compatiable with older systems. The Dot 3 & 4 brake fluid is also gylcol based but has higher dry boiling point in 450-500 deg range and also higher wet boiling point in the 290 - 320 deg range. Castrol's Dot 3&4 fluid is called Castrol GT LMA. The LMA stands for Low Mositure Affinity. Valvoline also has a Synetic Blend Dot 3&4 that is glycol based and has high boiling points. I plan to use the Valvoline as it has slightly higher boiling points than Castrol. I'll let everyone know how it goes I use my 63 Avanti as a daily driver and put 7000 miles on it last year so the car will not be sitting up for long periods of time. John Graham 63 R1 (R-2386)