Brake Fluid Comparo
The Department of Transportation (DOT) classifies brake fluids to defined specifications. These specifications relate to their boiling points and chemical composition, both of which are important. All currently availablebrake fluids are covered by one of the following specifications; DOT3, DOT4, DOT5 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 Brake Fluid (DOT 5) - 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 absorbwater. 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 thereforerequires 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 can vaporise when heated under heavy braking giving a disastrous effect on braking efficiency. DOT5 fluids are not recommended for motor sport applications PAG - Poly Alkyl 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 isconsiderably better than DOT 3 which it replaces. DOT 5.1 has higher specification still and is for fast road and occasional track day use.
DRY:401F -- WET:284F --- DOT3 MINIMUM
DRY:550F -- WET:284F --- PERFORMANCE Friction Z rated
DRY:550F -- WET:284F --- FORD HEAVY DUTY DOT 3
DRY:570F -- WET:284F --- WILWOOD 570
DRY:527F -- WET:302F --- AP RACING 551
DRY:446F -- WET:311F --- DOT4 MINIMUM
DRY:527F -- WET:347F --- VALVOLINE SYNPOWER DOT3 & DOT4
DRY:500F -- WET:356F --- DOT5.1 MINIMUM (GLYCOL BASE)
DRY:500F -- WET:356F --- DOT5 MINIMUM (SILICONE BASE)
DRY:536F -- WET:392F --- ATE SUPERBLUE/TYP200
DRY:601F -- WET:399F --- BREMBO LCF 600 PLUS
DRY:590F -- WET:410F --- AP RACING Super 600
DRY:626F -- WET:417F --- WILWOOD EXP 600
DRY:585F -- WET:421F --- Earl's Hypertemp 421
DRY:593F -- WET:420F --- MOTUL RBF600
DRY:620F -- WET:425F --- COBALT SUPER XRF [seem to no longer be available]
DRY:590F -- WET:518F --- CASTROL SRF
