Brake Fluid

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Stock brake fluid cannot handle the heat generated in driving your car on the track. The brake fluid will evaporate, and will now become more compressible, thus being unable to transfer the pressure applied to the master-cylinder by the pedal and booster, leading to brake fade. Normal DOT brake fluid handles temperatures up to 155C.

Many brake system problems are caused by old or inadequate brake fluid. It is very important to always use a high quality fluid with a high dry boiling point rating. There are also many theories as to what type of fluid should be used. The following information should help you better understand the differences regarding different types of fluids. A. Silicon based fluids: most all silicon-based fluids are very easy to compress. When using a fluid that is easier to compress it must be noted that a spongy pedal will likely be noticed. Many times the pedal will continually get even spongier as the fluid gets hotter. Another characteristic about silicone fluid, is its inability to mix with water. When the water begins to boil, it can cause the system to air lock. It should also be noted that silicon based fluid is not recommended in AFCO type master cylinders. B. Glycol based fluids: theses types of fluids are the most widely used and maybe the least understood. It is very important to understand that this fluid will mix with water. It is also important to remember that the boiling point of the fluid will drop considerably as the fluid mixes with the moisture in your brake system, making it extremely important to completely purge the entire system very often. No matter the type of fluid you choose, it is a good idea to change it often and never use fluid from an open container because of the potential for moisture.


Brake fluids: Introduction

Brake fluid boiling temperature are measured differently for "Dry" and "Wet". Brake fluid is subjected to very high temperatures, especially in the wheel cylinders of drum brakes and disk brake calipers. It must have a high boiling point to avoid vaporizing in the lines. This vaporization is a problem because vapor released into the lines is compressible and would result in an inability of the hydraulic fluid to transfer braking force. Quality standards refer to a brake fluid's "dry" and "wet" boiling points. Wet boiling point, which is usually much lower, refers to the fluid's boiling point after absorbing a certain amount of moisture. This is several percent, varying from formulation to formulation; in higher levels, the moisture itself can boil separately from the base fluid. Glycol-ether/dot three/dot four brake fluids are hygroscopic (water loving), which means they absorb moisture from the atmosphere under normal humidity levels. More modern fluids (e.g. silicone/DOT 5-based formulations), are hydrophobic, and can maintain an acceptable boiling point as they absorb moisture over the fluid's service life.

  • Castrol SRF 518F/270 C Wet 310C Dry Silicone Based Very Expensive
  • ATE Super Blue
  • ATE Typ200
  • Pentosin CHF11S PS
  • Motul 600

The Motul and ATE fluids listed above are all DOT4/DOT4+ compatible and should work well in most modern cars.

Automotive brake fluid has many responsibilities. Corrosion protection and lubrication of brake system components are only a portion of the role brake fluid must play.

All automobiles that have a hydraulic braking system must use brake fluid in order for the brake system to operate. The type of fluid used can depend on the type of vehicle and the demands of the vehicles brake system.

The two most common brake fluids used in the automotive industry are fluids that contain Polyalkylene Glycol Ether and fluid that contains Silicone or Silicium-based Polymer. Both Fluids are common but very different in regards to the manner in which they perform. Fluids containing Polyalklene Glycol Ether are more widely used and are the only fluids that should be used in racing brake systems.

Because brake systems may reach extreme temperatures brake fluid must have the ability to withstand these temperatures and not degrade rapidly.

Silicon 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-hydroscopic 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.

Polyglycol Ether Based fluids

luids containing Poly glycol ethers are regarded as DOT 3, 4, and DOT 5.1. These type fluids are hydroscopic 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 is 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).

Racing brake fluids always exceeds the DOT specifications for dry boiling points. Wet boiling points generally remain the same.

DOT 3 vs. DOT 4 & 5.1

AFCO's 570º brake fluid is a DOT 3 type fluid. However, it has a dry boiling point that is 52º higher than DOT 5.1 specifications, 124º higher than DOT 4 specifications and 169º higher than DOT 3 specifications. AFCO's 570º fluid meets or exceeds all DOT 3, 4, and 5.1 lubrication, corrosion protection and viscosity specifications.

AFCO's 570º racing fluid meets but does not exceed federal standards for wet boiling point specification; therefore, its classification is DOT 3. Because AFCO's 570º fluid is intended for use in racing type brake systems that undergo frequent fluid changes, exceeding federal standards for wet boiling points is of little concern. Racing brake fluids always exceeds the DOT specifications for dry boiling points. Wet boiling points generally remain the same.

Boiling points

The term boiling point when used regarding brake fluid means the temperatures that brake fluid will begin to boil. The minimum temperatures that brake fluids will begin to boil when the brake system contains 3% water by volume of the system. The temperatures that brake fluid will boil with no water present in the system.

Moisture in the brake system

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.

Brake fluids dry boiling point is more important then wet boiling point when used in a racing brake system. Passenger cars very rarely will undergo a brake fluid change making the wet boiling point more important. Racing brake system fluid is changed often and a system with fresh fluid will most likely not contain water. Because of this, racers should be concerned with the dry boiling point. Racing fluid exceeds DOT 3, 4, and 5.1 dry boiling point specifications. Never use silicone based fluids in racing brake systems. Using racing brake fluid will increase performance of the braking system. Never reuse fluid. º Never mix types or brands of brake fluid. Use smaller fluid containers that can be used quicker. If fluid remains in container be sure to tightly seal and do not store for long periods of time. Purge system (complete drain) and replace fluid often. Immediately replace master cylinder reservoir cap following any maintenance.

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