Practical Automotive Maintenance: Brake Fluid

Back to course

Watch This Course

$99 USD

-OR-

Or 8 easy payments of only $12.38 USD Instant access. Easy checkout. No fees. Learn more

Enroll Now
8 easy payments of only $12.38 USD

Course Access for Life

60 day money back guarantee

Brake Fluid

07.29

00:00	We've already covered how brake fluid is responsible for transmitting hydraulic pressure throughout the whole braking system.
00:06	After being pressurised by the master cylinders, it's then responsible for applying pressure to the brake caliper pistons.
00:12	One of the main properties of brake fluid that we care about is its compressibility.
00:16	That is, we want to minimise any volume change of the fluid when pressure is applied.
00:21	The lower the compressibility of our brake fluid, the less compliance we'll have, leading to a more direct action between the driver's brake pedal input and the brake caliper force.
00:31	To some extent, the chemistry of the different brake fluids affects its compressibility.
00:36	But when compressibility becomes the biggest problem is when the brake fluid is heated above its boiling point.
00:42	When this occurs, small air bubbles start to form inside the fluid, and air is much more compressible than the brake fluid and therefore results in a lot of compliance.
00:51	To the driver, this will result in more pedal travel being required to build a given amount of brake pressure.
00:57	This will often be described as a soft or spongy pedal, and it's not a nice sensation and quickly destroys the driver's confidence in the brakes.
01:05	In extreme cases, this can result in the pedal reaching its maximum mechanical travel before sufficient brake pressure is reached, with obvious dangerous consequences.
01:14	The source of the heat that causes the brake fluid to boil comes from the thermal energy absorbed by the brake calipers during braking events.
01:22	While the brake discs may operate at 800 degrees Celsius or more, the brake calipers operate at much lower temperatures.
01:29	However, this still makes controlling caliper temperatures and choosing a suitable brake fluid critical to avoid boiling.
01:36	One of the factors that has a large impact on the boiling temperature of our brake fluid is the amount of moisture it has in it.
01:42	Most brake fluids are hydroscopic, meaning that they readily absorb moisture.
01:46	We want to keep the moisture content of our brake fluid as low as possible as it makes it more susceptible to boiling.
01:53	The more moisture present in the fluid, the lower the boiling point is.
01:57	This is the reason that when we read the label of the brake fluid, we'll see that there is a dry and wet boiling point temperature listed.
02:04	We'll notice that the wet boiling point temperature is significantly lower than that of the dry.
02:09	This is also why when we open a new bottle of brake fluid, we'll notice that it has an airtight seal that must be broken before we can use it.
02:17	By sealing the brake fluid from the atmosphere, it prevents moisture from the humidity in the air from being absorbed.
02:23	This is also why using brake fluid from an opened bottle is not recommended.
02:28	The official recommendation is to throw away unused brake fluid once the bottle has been opened.
02:34	When it comes to classifying different types of brake fluid, this is achieved with their DOT ratings.
02:39	DOT stands for Department of Transport and for brake fluid, it lays out the official standards for approved brake fluids.
02:46	The full standard includes a long list of criteria, but the main thing we care about for motorsport is to choose a fluid that won't boil for our application.
02:56	DOT 3 is the lowest common standard and for a fluid to meet DOT 3, the dry boiling point must be above 205 degrees Celsius and the wet boiling point above 140 degrees Celsius.
03:08	To meet DOT 4, the dry boiling point must be above 230 degrees Celsius with a wet boiling point of 155 degrees Celsius and finally to meet DOT 5, it must have a dry boiling point of above 260 degrees Celsius and a wet boiling point of 180 degrees Celsius.
03:25	It's important to understand that just because a given brake fluid meets a given DOT rating doesn't mean it'll necessarily behave identically to another brake fluid from that same rating.
03:36	All it means is that the fluid meets or exceeds the minimum standards laid out by that particular DOT rating.
03:42	It would also be easy to think that choosing a brake fluid with the highest possible DOT rating for our car would be the best idea.
03:49	However, there's a little bit more to it than that.
03:52	While DOT 4 has a higher boiling point than DOT 3 for example, it may also be more prone to absorbing moisture, meaning it'll likely need to be replaced more often.
04:02	DOT 5 has a completely different chemistry to all the other types of brake fluid.
04:06	DOT 5 is silicon based, with everything else being based on glycol ether.
04:12	DOT 5 does have a higher boiling point than DOT 4, as well as not being hydroscopic, meaning it can't absorb moisture.
04:19	The downside of DOT 5 is that due to its different makeup, it's naturally much more compressible than DOT 3 or 4.
04:26	That makes DOT 5 fluid unsuitable for applications that require high hydraulic pressures, like motorsport.
04:32	As a result, DOT 5 brake fluid does not tend to be used in performance or motorsport applications.
04:38	DOT 5.1 is distinctly different to DOT 5.
04:42	Chemically speaking, DOT 5.1 is similar to DOT 4 in that it's glycol ether based.
04:48	But it also benefits from some differences that raise the boiling point to the DOT 5 standard.
04:53	This gives us the benefits of DOT 5 temperature rating without the poor compressibility of a DOT 5 fluid.
04:59	The inevitable downside of using DOT 5.1 fluid is that it's naturally a lot more expensive than the lower rated fluids.
05:07	This makes it important to distinguish between a DOT 5 and a DOT 5.1.
05:11	The 5.1 designated was introduced to help make the difference between the two types of brake fluid clear.
05:17	Another important point is that DOT 3, 4 and 5.1 are compatible with each other, meaning we can top up the system with any of these.
05:25	DOT 5 on the other hand does not mix with these other standards and can't be used interchangeably.
05:32	This is important to remember as if we have a car with DOT 5 fluid that we wanted to remove, simply flushing our system with a more suitable fluid is not a good idea.
05:41	That's because it's possible for pockets of the original brake fluid to remain in the system after the flush.
05:47	The only way to be confident in changing the brake fluid type is by stripping the brake system and ensuring that none of the previous brake fluid remains anywhere, which is obviously a time-intensive process.
05:58	Brake fluid should be thought of as a consumable item.
06:02	Whether it becomes degraded by absorbing moisture or overheating, it needs to be changed periodically.
06:07	This is due to diffusion, where moisture can pass through some materials such as some plastic fluid reservoirs and rubber-based caliper seals.
06:15	Brake fluid is also damaged by high temperatures, which can be recognized by a change in color.
06:20	When glycol ether-based brake fluid is new, it'll be a clearish yellow type color.
06:26	This will change to a brown color as the fluid breaks down from high temperatures.
06:30	In professional circuit racing for example, which is where we'll usually see the highest temperatures, it's not unusual to change the fluid between every session.
06:38	In less serious club racing, we should be checking our brake fluid with a small brake bleed at least after every event.
06:44	Note that we'll be covering the proper process of brake bleeding in a later module.
06:48	A performance road car will still likely benefit from a brake bleed annually, depending on the ambient environment and the components involved.
06:56	If we notice a difference in the pedal quality after a brake bleed, this can also be used as a guide for how often replacing the fluid is needed for our application.
07:04	In summary, brake fluid is used to transfer hydraulic pressure throughout the braking system.
07:09	The compressibility of the fluid is a primary concern in motorsport.
07:14	Brake fluid becomes compressible when it boils and boiling resistance is usually influenced by the moisture absorption.
07:21	Brake fluid should be treated as a consumable and replaced as often as necessary for our application.