My Technical Website on Brake Systems
All brake fluids absorb moisture, some faster than others (except silicone which is not recommended for anti-lock brake systems). Castrol SRF resists moisture contamination (non-hygroscopic) more than any other fluid we tested; therefore change intervals can be greatly extended. This reduces the effective cost over a season of racing. Many drivers say that they can run the same fluid all year long with only bleeding off the fluid in the calipers for each event. This way a can or two will last all year. Other fluids (hygroscopic type) require additional flushing of the system for each track event to maintain the lowest percentage of moisture and the highest boiling point.
FYI - The Castrol SRF is around $77/container versus $10-15/container for the rest.
Silicone Brake Fluids
Fluids containing Silicone are generally used in military type vehicles and because Silicone based fluids will not damage painted surfaces they are also somewhat common in show cars.
Silicone-based fluids are regarded as DOT 5 fluids. They are highly compressible and can give the driver a feeling of a spongy pedal. The higher the brake system temperature the more the compressibility of the fluid and this increases the feeling of a spongy pedal.
Silicone based fluids are non-hygroscopic meaning that they will not absorb or mix with water. When water is present in the brake system it will create a water/fluid/water/fluid situation. Because water boils at approximately 212º F, the ability of the brake system to operate correctly decreases, and the steam created from boiling water adds air to the system. It is important to remember that water may be present in any brake system. Therefore silicone brake fluid lacks the ability to deal with moisture and will dramatically decrease a brake systems performance.
Brake Fluid and Cold Temps.
Kinematic viscosities: All brake fluids (DOT 3, DOT 4 and DOT 5) must meet a minimum viscosity test of not less than 1.5 centistokes at 100° C (212° F) and must not be more than the following to meet their various classifications (the larger numbers indicate higher kinematic viscosities just like with motor oils).
DOT 3 1500 Centistokes at minus 40° C
DOT 4 1800 Centistokes at minus 40° C
DOT 5 900 Centistokes at minus 40° C
Higher kinematic viscosities means it "flows easier" at the cold temps.
A centistokes is 1 mm^2/s
MINIMAL boiling points for these specifications are as follows:
|| Dry Boiling Point || Wet Boiling Point
DOT 3 || 401ºF || 284ºF
DOT 4 || 446ºF || 311ºF
DOT 5 || 500ºF || 356ºF
DOT 5.1 || 518ºF || 375ºF
Poly Glycol Ether Based Brake Fluids
Fluids containing Poly glycol ethers are regarded as DOT 3, 4, and DOT 5.1. These type fluids are hygroscopic meaning they have an ability to mix with water and still perform adequately. However, water will drastically reduce the boiling point of fluid. In a passenger car this is not an issue. In a racecar it is a major issue because as the boiling point decreases the performance ability of the fluid also decreases.
Poly glycol type fluids are 2 times less compressible than silicone type fluids, even when heated. Less compressibility of brake fluid will increase pedal feel. Changing fluid on a regular basis will greatly increase the performance of the brake system.
FLUID SPECIFICATIONS All brake fluids must meet federal standard #116. Under this standard are three Department of Transportation (DOT) minimal specifications for brake fluid. They are DOT 3, DOT 4, and DOT 5.1 (for fluids based with Polyalkylene Glycol Ether) and DOT 5 (for Silicone based fluids).
Wet vs. Dry Boiling Point
WET BOILING POINT - The minimum temperatures that brake fluids will begin to boil when the brake system contains 3% water by volume of the system.
DRY BOILING POINT - The temperatures that brake fluid will boil with no water present in the system.
How does water get in there?
Water/moisture can be found in nearly all brake systems. Moisture enters the brake system in several ways. One of the more common ways is from using old or pre-opened fluid. Keep in mind, that brake fluid draws in moisture from the surrounding air. Tightly sealing brake fluid bottles and not storing them for long periods of time will help keep moisture out. When changing or bleeding brake fluid always replace master cylinder caps as soon as possible to prevent moisture from entering into the master cylinder. Condensation, (small moisture droplets) can form in lines and calipers. As caliper and line temperatures heat up and then cool repeatedly, condensation occurs, leaving behind an increase in moisture/water. Over time the moisture becomes trapped in the internal sections of calipers, lines, master cylinders, etc. When this water reaches 212º F the water turns to steam. Many times air in the brake system is a result of water that has turned to steam. The build up of steam will create air pressure in the system, sometimes to the point that enough pressure is created to push caliper pistons into the brake pad. This will create brake drag as the rotor and pads make contact and can also create more heat in the system. Diffusion is another way in that water/moisture may enter the system.
Diffusion occurs when over time moisture enters through rubber brake hoses. The use of hoses made from EPDM materials (Ethlene-Propylene-Diene-Materials) will reduce the amount of diffusion OR use steel braided brake hose with a non-rubber sleeve (usually Teflon) to greatly reduce the diffusion process.
DOT what?
DOT: Acronym for "Department of Transportation" -- an American federal agency or "Department of Transport" -- a British agency
DOT 3: This brake fluid has a glycol base. It is clear or light amber in color. Its dry boiling point is 401° minimum and wet boiling point of 284° minimum. It will absorb 1 to 2 percent of water per year depending on climate and operating conditions. It is used in most domestic cars and light trucks in normal driving. It does not require cleaning the system and it can be mixed with DOT 4 and DOT 5.1 without damage to the system. The problem with it is that it absorbs moisture out of the air and thereby reduces its boiling point. It can also damage the paint on a vehicle.
DOT 4: This brake fluid has a borate ester base. It is clear or light amber in color. Its dry boiling point is 446° minimum and wet boiling point of 311° minimum. It is used in many European cars; also for vehicles in high-altitude, towing, or high-speed braking situations, or ABS systems. It does not require cleaning the system and it can be mixed with DOT 3 without damage to the system. The problem with it is that it absorbs moisture out of the air and thereby reduces its boiling point. It can also damage the paint on a vehicle.
DOT 5: This brake fluid generally has a silicone base. It is violet in color. Its dry boiling point is 500° minimum and has no wet boiling point in federal DOT 5 specifications. It is used in heavy brake applications, and good for weekend, antique, or collector cars that sit for long periods and are never driven far. It does not mix with DOT 3, DOT 4, or DOT 5.1. It will not absorb water and will not damage the paint on a vehicle. It is also compatible with most rubber formulations. The problem with it is that it may easily get air bubbles into the system that are nearly impossible to remove, giving poor pedal feel. It is unsuitable for racing due to compressibility under high temperatures. If as little as one drop of water enters the fluid, severe localized corrosion, freezing, or gassing may occur. This can happen because water is heavier and not mixable with silicone fluids. It is unsuitable for ABS.
DOT 5.1: This brake fluid has a borate ester base. It is clear or light amber in color. Its dry boiling point is 500° minimum and wet boiling point of 356° minimum. It is used in severe-duty vehicles such as fleets and delivery trucks, towing vehicles, and racecars. It can be mixed with DOT 3 or DOT 4 without damage to the system. It maintains higher boiling point than DOT 3 or DOT 4 fluids due to its higher borate ester content. It is excellent for severe duty applications. The problem with it is that it costs more than other fluids and there is limited availability. It also absorbs moisture out of the air and thereby reduces its boiling point. It can also damage the paint on a vehicle.
What causes a mushy pedal?
DOT 5 fluid is not hygroscopic, so as moisture enters the system, it is not absorbed by the fluid, and results in beads of moisture moving through the brake line, collecting in the calipers. It is not uncommon to have caliper temperatures exceed 200° F, and at 212° F, this collected moisture will boil causing vapor lock and system failure. Additionally, DOT 5 fluid is highly compressible due to aeration and foaming under normal braking conditions, providing a spongy brake feel.
Brake Bleeding/Flushing
One thing that is ALWAYS true never let the TMC (master cylinder) on an ABS, traction control (TCS) or electronic stability program (ESP) car run dry. You'll never get the air bubbles out again by hand. To be honest there is more than one right way and if you found something that works, why fix what's not broken.
Just FYI-
At the factory this is how it's done. They do it all one shot.
First you mount the Evac-Fill head unit to the reservoir then all air is evacuated from the system, creating a vacuum. Then fluid is forced through the system at high pressure.
Then the car is delivered and people complain about mushy pedal!
Just some more stuff:
I e-mailed a Tech at Ford I used to work with on the proper bleed sequence. Since techs do these all the time and engineers don't I'll take his advice.
This is it:
Doing nearest or most distant doesn't matter. What matters is if the brake system is a diagonal or front-rear system.
Quick definition: diagonal means that one circuit in the master cylinder feeds the front driver's side and the rear passenger side. The other circuit the front passenger's and rear driver's. Both circuits are of equal volume. Front-Rear means that one circuit supplies the front calipers, while the other the rear calipers. The Primary circuit (front caliper circuit) is of greater volume.
All you need to do is make sure you do the circuits together. For instance, on most passenger cars it's a diagonal system. So you want to do the driver's front and passenger's rear together, which you do first is not all that important. But he does agree that the tradition is to do the rear first. Or best to get a friend and do both at the same time. And take your time (he told me to make sure I added this).
Same holds true for the Front-Rear systems. Do fronts together and rears together. Usually only trucks, SUVs and very heavy front bias cars (Ford Crown Vic) have a Front-Rear system.
Just FYI- Diagonal is used so that if one circuit fails the vehicle is still stoppable in a stable manner as at least one front and rear wheel is braking and on opposite sides. Front-Rears are used on heavy, front-bias cars require a lot of volume up front.
ABS Bleeding:
Do 3 normal bleeds and then do
An ABS stop if you still feel that air might be trapped. Then do 1 more
Bleed. The ABS stop would have flushed the air out.