Mr.F
Inactive
Further to some discussions regarding suitability of various brake fluid formats and use of silicone fluid, here is a recent Technical Bulletin from Millers Oils on the subject (they are advertising their new 300+ racing brake fluid, of course, but the info is useful!):
"There is a degree of confusion regarding the specification of brake fluid and this article sets out to clarify the situation.
The Department of Transportation (DOT) classifies brake fluids to defined specifications. These specificationsrelate to their boiling points and chemical composition, both of which are important. Automotive brake fluidsare covered by one of the following specifications; DOT3, DOT4, DOT 5 and DOT5.1.
The laws of thermo-dynamics dictate that the energy from motion is turned into heat through friction. A braking system only works efficiently if the fluid remains incompressible. If the brake fluid boils, it turns to gas, which is compressible and the braking system becomes "spongy" or in extreme cases fails completely.
A brake system is not perfectly sealed and moisture can get into the system and be absorbed by the fluid.
The effect is to reduce the boiling point of the fluid, which reduces the efficiency of the braking system, as described above.
The DOT specify two reference tests for brake fluids,
Dry boiling point - the boiling point of fresh fluid
Wet boiling point - the boiling point once the fluid has absorbed moisture (representing brake
fluid after time spent in a real situation).
There are two main types of brake fluids.
DOT 3, DOT 4 and DOT 5.1 which are based on glycol (Poly Alkylene Glycol or PAG)
DOT 5, which is based on silicone.
Note the two types of fluid are not compatible and must not be mixed in a braking system.
Silicone based DOT 5 was originally introduced to give higher temperature performance over glycol
DOT 4. Silicone fluid also has other advantages, it does not damage paintwork and it does not absorb water. However, silicone fluid is a poor lubricant and does not lubricate ABS pumps as well as PAG fluids.
It is also more compressible than PAG fluids, which can result in a sluggish or spongy pedal. It therefore requires special design considerations in baking systems. Further, because it does not absorb water, anywater remains as globules, which can pool in low spots in the system and cause corrosion. This water canvaporise when heated under heavy braking giving a disastrous effect on braking efficiency.
DOTs fluids are not recommended for motor sport applications.
PAG - Poly Alkylene Glycol Brake Fluids (DOT 3, 4 and 5.1)
Glycol-based DOT 4 fluid is the current mainstream brake fluid, and you will see that the specification is considerably better than DOT 3 which it replaces. DOT 5.1 has a higher specification still and is intendedfor fast road and occasional track day use.
Millers Racing Brake Fluid 300 Plus
Millers Racing Brake Fluid is a very high boiling point glycol based brake fluid specifically developed for motor sport application. It isnominally designated DOT 4, (as are all racing brake fluids) but it'sperformance exceeds DOT 5.1 on both wet and dry boiling points.
In addition it has extremely good "compressibility" characteristicsto ensure excellent pedal feel."
"There is a degree of confusion regarding the specification of brake fluid and this article sets out to clarify the situation.
The Department of Transportation (DOT) classifies brake fluids to defined specifications. These specificationsrelate to their boiling points and chemical composition, both of which are important. Automotive brake fluidsare covered by one of the following specifications; DOT3, DOT4, DOT 5 and DOT5.1.
The laws of thermo-dynamics dictate that the energy from motion is turned into heat through friction. A braking system only works efficiently if the fluid remains incompressible. If the brake fluid boils, it turns to gas, which is compressible and the braking system becomes "spongy" or in extreme cases fails completely.
A brake system is not perfectly sealed and moisture can get into the system and be absorbed by the fluid.
The effect is to reduce the boiling point of the fluid, which reduces the efficiency of the braking system, as described above.
The DOT specify two reference tests for brake fluids,
Dry boiling point - the boiling point of fresh fluid
Wet boiling point - the boiling point once the fluid has absorbed moisture (representing brake
fluid after time spent in a real situation).
There are two main types of brake fluids.
DOT 3, DOT 4 and DOT 5.1 which are based on glycol (Poly Alkylene Glycol or PAG)
DOT 5, which is based on silicone.
Note the two types of fluid are not compatible and must not be mixed in a braking system.
Silicone based DOT 5 was originally introduced to give higher temperature performance over glycol
DOT 4. Silicone fluid also has other advantages, it does not damage paintwork and it does not absorb water. However, silicone fluid is a poor lubricant and does not lubricate ABS pumps as well as PAG fluids.
It is also more compressible than PAG fluids, which can result in a sluggish or spongy pedal. It therefore requires special design considerations in baking systems. Further, because it does not absorb water, anywater remains as globules, which can pool in low spots in the system and cause corrosion. This water canvaporise when heated under heavy braking giving a disastrous effect on braking efficiency.
DOTs fluids are not recommended for motor sport applications.
PAG - Poly Alkylene Glycol Brake Fluids (DOT 3, 4 and 5.1)
Glycol-based DOT 4 fluid is the current mainstream brake fluid, and you will see that the specification is considerably better than DOT 3 which it replaces. DOT 5.1 has a higher specification still and is intendedfor fast road and occasional track day use.
Millers Racing Brake Fluid 300 Plus
Millers Racing Brake Fluid is a very high boiling point glycol based brake fluid specifically developed for motor sport application. It isnominally designated DOT 4, (as are all racing brake fluids) but it'sperformance exceeds DOT 5.1 on both wet and dry boiling points.
In addition it has extremely good "compressibility" characteristicsto ensure excellent pedal feel."