Brake Fluid
- Thread starter 964RS
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944 man
Active member
Higher than DOT4 but lower than DOT5.1. The only advantage is that it isnt hygroscopic, so theres no lower 'wet' boiling point. Its really only suitable for new cars or complete new systems though: trying to add DOT5 fluid to an existing system is dangerous, as complete brake failure is the likely outcome.
sawood12
New member
Well my philosophy is I don't want the brake fluid to be the weakest link in the braking system. If your fluid boils you loose all braking, if your pads or discs overheat you still get some braking, so I use ATE blue - which is very good and relatively cheap and have upgraded the brakes. Sounds like a more expensive option on the face of it, but at the cost of more regular replacements of very expensive brake fluid it doesn't take long for it to be cost effective. I know i'm nowhere near a good enough driver to reach the capabilities, and if I were to become that good then i'd opt for improved cooling options.
tr7v8
New member
Wrong.One of the DOT tests is that ALL brake fluid can intermix. However you get no advantages from intermixing brake fluids. DOT 5 was originally designed for the American military. And doesn't damage paintwork. Silicone does have some compressibility issues,some cars seem worse than others, people have experienced a long pedal somewhat like spongy brakes. As for cleaning, obviously a completely rebuilt system is clean the other option is to flush using meths.
Hilux
New member
Dot 5 cannot be mixed with other fluids all others can and (see the last paragraph) should only be used in a completely new system. ****************** Shamelessly stolen from Moss who specialise in older cars. The Real Story: Conventional vs Silicone Brake Fluid Today's current Lockheed "Universal", Girling "LMA", and silicone brake fluids are so vastly superior to the old Girling "Green" and "Crimson", and Lockheed "Heavy Duty" fluids originally specified for most of our older British sports cars, that it would make no sense to use the older types today, even if they were still available. The most notable advances have been in raising boiling points, improved compatibility with each other, and reducing moisture absorption. The main function of brake fluid is to transmit pedal movement to the brake pads and shoes. To do this efficiently, brake fluids must be non-compressible. They must also not boil at the highest operating temperatures encountered, thicken or freeze at cold temperatures, not corrode or chemically react with any materials in the hydraulic system, and not decompose or form sludge, gum, or varnish at any temperature. They must lubricate internal moving parts, flow easily through small passages, have a long and stable shelf life, and be compatible with other brake fluids. Brake fluids are classified by their chemical type and boiling points. The different chemical bases currently used are polyalkylene glycol ether (commonly called glycol), silicone, and mineral oil. (Of these, mineral oil doesn't concern us, as it is used in very few cars, none of which Moss Motors deals with.) D.O.T. 3 and D.O.T. 4 brake fluids are glycol-based, while silicone-based fluids are classified as D.O.T. 5. To further confuse matters, there is now a D.O.T. 5.1 brake fluid which has a diethylene glycol-ester base, with properties similar to D.O.T. 4, but with enhanced performance characteristics. These D.O.T. (Department of Transportation) specifications also indicate minimum boiling points. It is important to note that these D.O.T. specifications are performance specifications, not material specifications; for example, D.O.T. 5 sold in Europe is not silicone-based, as it is in the USA. In the good old days, little good could be said of brake systems. Warnings such as, "as the cups in the master cylinder are pure rubber; it is imperative to use only the recommended fluid. Any other fluid may be dangerous" were common. Such strong concerns were very valid in the 1950s, much less so now, even for 1950's vintage cars. The reasons for this lessened worry about our hydraulic systems "turning to goo" if the wrong fluid is used is that: 1.) pure rubber hydraulic seals are no longer made for our cars, and 2.) D.O.T. 3, 4, and 5 brake fluids are safe to mix, and are compatible with the seals now available. While these brake fluids are safe to mix, mixing them is not recommended. When brakes are applied on a moving car, the kinetic energy of the car is turned into heat. The faster the car is moving and the faster it is stopped, the more heat is produced. Some of this heat soaks into the brake fluid. In the late 1940s, brake fluid with a boiling point of 235° F was considered adequate. By about 1957, the lowest S.A.E. specification was for a minimum boiling point of 302° F for cars with drum brakes. Disc brakes presented new problems. In stopping faster (and often heavier) cars more quickly, they generated even more heat which had to be dissipated, with an accompanying requirement for brake fluid with even higher minimum boiling points. Improvements in brake lining materials, brake drum and rotor design and metallurgy have also had a similar effect; improvements in braking efficiency require improvements in brake fluids. To handle these higher temperatures, improvements were also made in wheel cylinder and brake caliper seal design and materials. Brake fluids must not be allowed to boil for two reasons: 1) The brakes won't work due to the vapor bubbles being compressible. 2) Physical and chemical properties of the brake fluid may change due to the "lighter" components boiling off. Glycol-based brake fluids in particular, are hydroscopic (moisture absorbing), some more so than others. When water is absorbed, the boiling point is sharply lowered. This occurs because water boils at only 212° F. When brake fluid is mixed with water, the boiling point of the mixture is less than that of the "dry" brake fluid. See chart for D.O.T. minimum boiling point specifications. [font=verdana,arial,helvetica]D.O.T. Minimum Boiling Points (degrees Fahrenheit):[/font] [font=verdana,arial,helvetica]D.O.T. 3[/font] [font=verdana,arial,helvetica]D.O.T. 4[/font] [font=verdana,arial,helvetica]D.O.T. 5[/font] [font=verdana,arial,helvetica]Dry[/font] [font=verdana,arial,helvetica]401[/font] [font=verdana,arial,helvetica]446[/font] [font=verdana,arial,helvetica]500[/font] [font=verdana,arial,helvetica]Wet[/font] [font=verdana,arial,helvetica]284[/font] [font=verdana,arial,helvetica]311[/font] [font=verdana,arial,helvetica][/font] 356 *This is the required min. specification, but does not reflect actual performance of silicone-based fluids. Since D.O.T. 5 is non-hydroscopic, its actual "wet" boiling point is essentially the same as its dry boiling point. However, a brake system using silicon-based fluid with water present in the system would show an effective boiling point of 212 degrees due to the free water. Water contamination also leads to corrosion of brake pipes, wheel cylinders, calipers, and master cylinders, resulting in pipe leaks, "frozen" cylinder pistons, accelerated seal wear, and the formation of sludge. Silicone fluids avoid these problems by being non-hydroscopic (not moisture-absorbing), while glycol fluids can absorb as much as 6% water just by being in a "sealed" automotive hydraulic system for a few years. This moisture is generally absorbed from the air. Some moisture even works its way into brake hoses. Most comes from master cylinder cap vents and resultant condensation in the air space above the fluid, and from allowing cans of brake fluid and master cylinders to remain open to the atmosphere for too long. Silicone fluids absorb a tiny amount of moisture (on the order of 280 parts per million, or .0028%) and then absorb no more. Silicone fluids, in addition to having high boiling points and being non-hydroscopic, do not damage paint as do glycol fluids. This is of particular importance in regard to show cars where a spill or leak of glycol fluid can have seriously ugly results. There are, however, some disadvantages to silicone fluids. They are slightly compressible, particularly near the higher end of their temperature range. While this is of absolutely no consequence for normal street use, this is why silicone fluids are not used in race cars. (Conversely, racing hydraulic fluids should not be used in street cars. This is because, although racing brake fluids have high dry boiling points, most are highly hydroscopic, and have relatively very low wet boiling points. They would probably work extremely well if you were to change the fluid every week or so.) Because air bubbles do not regularly dissipate in silicone brake fluid, special care must be used to prevent them from forming during pouring and bleeding operations. The best way to bleed a silicone fluid system is with an Eezibleed Kit (Moss #386-860). Lacking that, bleed with slow pedal strokes, avoiding "pumping" the pedal. It may be necessary to bleed the system again in a day or so if there were any air bubbles which wouldn't bleed out the first time. A newly rebuilt and scrupulously clean brake system filled with silicone fluid should outlast a system filled with glycol fluid by several times. There is little advantage in adding silicone fluid to a system which contains even small amounts of contaminants. Merely bleeding the system is not enough, as there will be pockets of old fluid and sludge which will not bleed out. Silicone fluid tends to concentrate any residual glycol fluid, moisture and sludge, into slugs, instead of allowing their dispersal throughout the fluid, as does glycol fluid. This can lead to relatively severe but localized problems, rather than the more general system deterioration experienced with old moisture-laden glycol fluids. This may be a factor in reports of leakage when silicone fluid is used in non-rebuilt systems which had been used with glycol fluid. A "new" system full of silicone fluid will require very little maintenance for years.
sawood12
New member
Firstly Silicone fluids are still hydroscopic - just not as much as Glycol based fluids which are quite aggressively hydroscopic - and the braking systems on cars is vented to atmosphere because as your brake linings wear the fluid level drops therefore air is being drawn into the system and moisture from that air absorbed into the fluid. As far as i'm aware no current car manufacturers for any car no matter how fast or exotic don't spec Silicon brake fluid as OEM standard (willing to be told otherwise). Silicon fluids have a whole crop of issues themselves which often outweighs the advantage of higher boiling point (which most people don't need) and the fact they are less corrosive to paint (a bit of due care and attention here mitigates this risk). For example i've read that Silicone fluids can mix more readly with air - they recommend that you let a tin of fluid stand and settle for a few hours before introducing into your system to allow air mixed during transit to settle out. And we all know air in fluid is a big no no. First golden rule is to replace your brake fluid regularly - every 2 years. If you are boiling DOT 4 brake fluid then more is needed than a simple change to DOT 5 or 5.1. You should consider improved cooling and/or brake upgrades to manage heat better and to look to improve your braking technique. Decent braking technique can result in better braking and less heat generation. A decent DOT 4 should be easily adequate for all but the heaviest track work. Certainly more than adequate for a predominantly street car. Why make life more complicated than it already is. Stick to DOT 4 and improve other aspects of your braking system. The best way to avoid boiling brake fluid is prevention - you may only get one chance.
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tr7v8
New member
I never ADVISED mixing them & it isn't a good idea! But you said "Not really no, because the two fluid types don't mix." & that is incorrect as part of the DOT tests is all versions will be intermixable. This is the salient line "Modern cars have split hydraulic circuits to ensure against total hydraulic failure. As a general rule, brake fluids with different DOT ratings should not be mixed, although all DOT fluid is compatible, (this is part of the DOT specification). This is because it will dilute and reduce the properties of the higher specification DOT fluid, or in the case of mixing of glycol with silicone fluid may cause corrosion due to trapped moisture." If I was stuck in the middle of nowhere needing brake fluid it could come in useful at that point! DOT 5.1 was developed for ABS use & can cause problems in older systems or certain makes.
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