There are several different types and brands of brake fluid and choosing the fluid that fits your vehicle and driving style can be confusing. First let's look at the common ways we classify these fluids. The Department of Transportation (DOT) classifies fluids based on their boiling points and chemical composition, both of which are important. Let's look at chemical composition first. Generally, there are two types of brake fluids: glycol, and silicone based; most brake fluids are glycol based which is short for Polyalkylene Glycol Ether. These fluids are hydroscopic (absorbs water) and are classified as DOT 3, DOT 4 and confusingly, DOT 5.1. Silicone based fluids are not hydroscopic and are classified as DOT 5. For reasons listed (far) below, these silicone based products are not really compatible with our brake system and should be avoided.
OK, so we have DOT 3, DOT 4, DOT 5 and DOT 5.1 and we know DOT 5 is silicone based and the rest are glycol based. The rest of the classification has to do with WET and DRY BOILING POINTS. Why are boiling points so important? There's no avoiding it, today's brakes get HOT, tomorrow's "brakes" may not, but that's another story. They get hot because the laws say so, the laws of thermal dynamics that is, but that's another story too. Because brakes have to get hot to stop the car and hydraulic systems only work if the fluid in the system remains uncompressible, it is imperative that the fluid not boil when the brakes do their intended job. If the fluid boils, it turns to gas which is compressible and the brake pedal gets that not-so-good "spongy" feeling. In bad cases, this will result in complete brake failure with the driver franticly pumping the brake pedal to the floor--the link between the pedal and the pads broken with the boiling fluid.
We understand why the boiling point is important and in a perfect world, we could just choose the fluid with the highest boiling point needed for our driving style and be done with it. Unfortunately, our brake systems are not perfectly sealed and air seeps into the system . Over time the fluid will absorb the moisture in the air which reduces the boiling point of the fluid. The DRY boiling point referees to the boiling point of fresh fluid and the WET boiling point reefers to the minimum boiling point once the fluid has absorbed moisture. Listed in the table below, are the minimum dry/wet specifications for each DOT level.
|Boiling Point||DOT 3||DOT 4||DOT 5 (silicone-based)||DOT 5.1 (non-silicone based)|
These are only the minimum levels, the manufacture may choose to exceed any or both of these specifications. Example: Ford Heavy Duty Brake Fluid is classified as DOT-3 although it has a dry boiling point of over 500F, but its wet boiling point is under 311F so it has to be classified as a DOT-3 fluid.
The problems with silicone based fluids
First, silicone based fluids are resistant to absorbing water, this is why their wet boiling points are so high. Instead, the water eventually pools in the low spots of the brake system and causes rust. Second, silicone based fluids are not compatible with our ABS system because it doesn't lubricate the ABS pump like a glycol based fluid. Third, putting this fluid in our system will cause the seals in the caliper and master cylinder seals to malfunction and will need replacing.
Brake pads are held by the caliper--sandwiched on either side of the rotor; when pressed together, the friction causes the car to stop. The pad is made up of friction material bonded to a backing plate. The caliper piston will push on the backing plate which applies the friction material to the rotor. This friction material eventually wears out causing us to replace the whole pad (friction material and backing plate) since they are bonded together. The service manual recommends replacing the pads when the friction material thickness is less than 1.7mm.
Pad Friction Material
We ask our pads to perform in a wide temperature range, wear well and not squeal, dust or eat our rotors. Like everything else, there are pads that excel in some of these categories, but at the expense of the other categories. The pad friction material typically comes in three forms:
These pads are well-suited for street driving because they wear well and are easy on the ears, dust and rotors. The pads work well when they are cold which is important in street driving. The drawback is they don't work well when they get really hot. Like all pads, they will fade when they reach a certain temperature; this fade temperature is lower with organic pads then the other types listed below.
This is an excellent compromise between street and track. These seem to be the pad of choice for those of us who both track and street our cars. These pads, used properly, will last several driver's school events and will get us to and from these events with minimal problems. They won't work as well as organic pads when they are cold, so care has to be taken with those first few stops. They fade at a higher temperature then organic pads. Sometimes the weak link in these pads is the bonding material that holds the linings to the backing plate. I've seen these pads delaminate when subjected to extreme heat and abuse.
These pads are typically reserved for racing. They squeal and dust like crazy, are hard on rotors and don't work well when cold. But they stop well, even at extreme temperatures.