Vehicles braking systems use hydraulics to transfer the braking action from the drivers pedal or lever to the master cylinder and then finally on to the brake caliper. The hydraulic system allows fluid pressure (or braking force) to be transmitted from one end of the braking system to the other, pushing the brake pads against the brake disc and decelerating the vehicle. Sounds simple right?
In fact it is far from that, a vehicles hydraulic braking system is a carefully balanced precision system that needs respect for cleanliness and regular servicing. Most importantly, when servicing it is vital that the correct type of brake fluid to be used.
There are two basic groups of brake fluid, Glycol fluids and silicone fluids (there are also one or two cars in France using mineral fluids but we leave that aside for the purpose of this article as this is very uncommon indeed).
Glycol fluids are the most common and used in 99.9% of motor vehicles in various grades. They are named by their DOT (Department of Transport) coding and are either DOT 3, DOT 4 or DOT 5.1. In general, the higher the number the higher the typical brake fluid boiling point. Although there are some exceptions to this rule, some high performance/racing DOT 4 fluids are highly refined giving them a significantly higher boiling point than a regular DOT5.1 fluid. For instance, EBC BF307+ racing fluid is a DOT 4 fluid with a dry boiling point of 307 degrees C, which is significantly higher than a typical DOT 5.1 fluid with a dry boiling point of only 260 degrees C. Hence when attempting to determine if one brake fluid is superior to another you must read the technical specification and compare the quoted boiling points.
Every brake fluid will specify two boiling points, the dry boiling point and the wet boiling point. At this point you may be thinking a ‘dry’ fluid sounds ridiculous since surely all brake fluids are wet, but what we mean by a ‘dry’ fluid is that the fluid does not contain any water. Hence a ‘wet’ fluid does contain water.
Crucially, all glycol fluids are hygroscopic which means that they absorb moisture from the surrounding atmosphere like a dry sponge absorbs water. This means that a glycol fluids water content will gradually increase over time as it absorbs moisture from the surrounding air. That’s why it’s absolutely critical to never use brake fluid from opened containers. On the other hand silicone based DOT 5 fluid is hydrophobic and this means the fluid repels moisture in the surrounding air.
The ‘dry boiling point’ can be considered the brake fluids boiling point when brand new, under optimal conditions and with minimal water content. However, over time glycol fluids degrade as they gradually absorb water from the air thus becoming a ‘wet’ fluid. The quoted ‘wet boiling point’ can be considered a worst-case scenario for a brake fluid nearing the end of its life and in need of replacing. The reason a wet boiling point is significantly lower than the dry boiling point is because water has a much lower boiling point (100 degrees C) than the brake fluid itself so the higher the water content in the brake fluid the lower the overall boiling point becomes. Leave any glycol fluid open to the atmosphere for more than a few days and its boiling point will fall to the ‘wet boiling point’ level.
Water in the brake fluid or a low boiling point is undesirable because as the brakes get hot during use, some of the heat is transferred into the brake caliper and thus into the hydraulic fluid. During rigorous braking temperatures can become so high that the brake fluid boils, introducing bubbles of gas into the hydraulics. This is very bad news since the bubbles of gas increases the overall compressibility of the brake fluid leading to a spongey pedal/lever and in extreme cases can lead to complete brake failure. Many track day drivers will recall a hair-raising moment when their brake pedal or lever bottomed out completely after boiling their brake fluid.
Of course, the other reason water in the brake fluid is bad news is because this then allows components in the hydraulic system to corrode, which over time can lead to replacing parts and massive repair bills.
You might also expect that once the brake fluid reservoir cap is screwed on tight there is no chance of water getting into the vehicles brake fluid, but in fact plastic is porous to a small degree and hence the rubber brake lines and plastic reservoir will allow a tiny amount of moisture to pass through it. The fact that glycol fluids gradually degrade in quality over time as water is absorbed is the reason that hydraulic braking systems must be periodically flushed through with new fresh brake fluid in order to maintain good braking performance. On a typical road car, replacing the fluid every 3 years is considered the minimum but the conditions under which the vehicle is driven also play a factor here. It is not uncommon for track cars to replace the fluid every 3-6 months, or racers to replace fluid every single event!
As a rule of thumb, if you are driving enthusiastically and begin to experience brake fade your first thought should be to when you last changed the fluid. If it has been a while then your first try should be to bleed the hydraulics through with a brand new high quality fluid. Since new brake fluid is significantly less expensive than buying new pads, this is a sensible first attempt at resolving the issue.
In general the higher the boiling point of the fluid used, the better. Although you should note that if the body of the brake caliper itself is exceeding 250 degrees C then you have a serious problem, not even Formula 1 brake calipers run above 250 degrees C. You might consider using brake caliper paint to determine the calipers max operating temperatures and then duct appropriate cooling to the brakes as necessary to manage these temperatures.
A LITTLE ON THE DIFFERENT FLUID TYPES
DOT 3 is a basic brake fluid that has a moderate water content and is not common in modern vehicles.
You can think of DOT 4 as a more refined & higher grade DOT 3 having a lower water content. The majority of modern vehicles use DOT 4 as this grade of fluid is a good compromise between cost and performance and is more than suitable for your typical daily driver, but beware, open a DOT 4 fluid and leave for a couple of weeks and your fluid will degrade to DOT 3 or worse. For this reason EBC only supply brake fluid in top-up size bottles that are individually sealed air tight, because once opened a bottle of fluid cannot be re-used and must be disposed of. Some workshops purchase brake fluid in large gallon containers but EBC refuse to sell fluid in such vast quantities because unless your doing 10+ cars in a short period the fluid will degrade resulting in the majority of it being wasted.
DOT 5.1 is yet a further refined glycol fluid with lower water content than a regular DOT 4 or DOT 3 fluid. DOT 5.1 fluids are popular as an upgrade on DOT 4 for higher performance vehicles since DOT 4 and DOT 5.1 are both glycol base and are therefore compatible. EBC however does not offer a DOT 5.1 fluid since EBC BF307+ racing fluid is what’s known as a Super DOT 4 fluid and has a higher boiling point than DOT 5.1 anyway.
There are several other things to note about glycol brake fluids. First they are toxic so never drink or ingest them and secondly they are very caustic so when working with glycol brake fluids keep them fluid and contaminated rags or your fingers away from vehicle paintwork. Wash off any spills quickly with soapy water.
Silicone DOT 5 fluids are another story, they are not toxic (although it is never good sense to drink any such fluid) and not caustic. For this reason some classic car and motorcycle builders use silicon fluids to avoid paintwork and plastic damage. Harley Davidson for example use silicone fluids in some bikes.
Mixing glycol and silicone fluids is definitely not advised as it causes an interface which means the pressure transfer does not happen effectively and certain caliper and cylinder hydraulic seals which may be made from viton or nitrile rubber are designed for one type of fluid but are destroyed by the other.
Always check the spec of brake fluid required for your vehicle on master cylinder or in the manufacturers handbook.