oilman Posted December 13, 2010 Share Posted December 13, 2010 Brake fluid... Bit of a mystery topic!To help dispel some myths and for some good solid general info on the mysterious world of brake fluids I decided to contact Millers Oils up in West Yorkshire. Their Technical Director, Martyn Mann was on hand to give us some useful info… below is Martyn's article on brake fluids.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 specifications relate to their boiling points and chemical composition, both of which are important. All currently available brake 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 specifies 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, Super DOT4* and DOT 5.1 which are based on poly glycol compounds.* DOT 5, which are 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 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 braking systems. Further, because it does not absorb water, any water 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. POLY 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 higher specification still and is for fast road and occasional track day use. It has a similar spec to DOT4 for the boiling point (>260) but is a lot lower viscosity @-40C typically 900 centistokes (compared to 1500 - 1800 centistokes for DOT 4 and super DOT 4).Listed in the table below, are the minimum dry/wet boiling point specifications for each DOT level.BOILING POINT:DOT 3 - 205°C (dry) / 140°C (wet)DOT 4 - 230°C (dry) / 155°C (wet)DOT 5 (silicone) - 260°C (dry) / 185°C (wet)DOT 5.1 (PAG) - 260°C (dry) / 185°C (wet)Super Dot4 * - 300°C (dry) / 195°C (wet)(racing brake fluid) * Super DOT4: The main difference between DOT 4 and Super DOT 4 is the dry boiling point. Normal Dot4 is >260C whilst Super DOT 4 is more like >310CWith thanks to Martyn Mann - Technical Director Millers Oils.CheersGuy.Opie Oils Link to comment Share on other sites More sharing options...
johngreen537 Posted December 13, 2010 Share Posted December 13, 2010 Cracking! DOT5.1 here I come Link to comment Share on other sites More sharing options...
johngreen537 Posted December 13, 2010 Share Posted December 13, 2010 On another note, fon't suppose you have a similar article on transmission fluid....? Link to comment Share on other sites More sharing options...
oilman Posted December 13, 2010 Author Share Posted December 13, 2010 We do, will post a new thread now.CheersGuy Link to comment Share on other sites More sharing options...
johngreen537 Posted December 13, 2010 Share Posted December 13, 2010 Magic! Cheers Guy! Link to comment Share on other sites More sharing options...
oilman Posted December 13, 2010 Author Share Posted December 13, 2010 New thread posted.CheersGuy Link to comment Share on other sites More sharing options...
wezza9 Posted December 13, 2010 Share Posted December 13, 2010 so super Dot 4 is for racing then is it ?i do know that you should change your brake fluid every 3 years or sooner if the car is used hard because the brake fluid is hygroscopic. I wonder how many people are driving there 20 year old car on the road which has had only 1 brake fluid change in its life i normally do it every car i get asap and fit braded brake lines too ( i had this car six months still not done it but lines ordered on wed ) Link to comment Share on other sites More sharing options...
oilman Posted December 13, 2010 Author Share Posted December 13, 2010 HiYes, I'd go for the Super DOT4 in a drag car.It's definately worth changing pretty much all fluids when you get a car, people rarely bother with the brake fluid, but other fluids can be neglected or they aren't what they told you they used.CheersTim Link to comment Share on other sites More sharing options...
wezza9 Posted December 14, 2010 Share Posted December 14, 2010 what about racing its not for a honda ? but a car i am building for some one i work with to race Link to comment Share on other sites More sharing options...
oilman Posted December 15, 2010 Author Share Posted December 15, 2010 When it comes to racing, super DOT4 is the way to go in any car pretty much Link to comment Share on other sites More sharing options...
Recommended Posts
Archived
This topic is now archived and is closed to further replies.