Machinery Lubrication magazine published by Noria Corporation
Issue link: https://www.e-digitaleditions.com/i/730290
Air Air contamination within a braking system can occur in a variety of ways. The leading cause is poor bleeding of the system. Air may also enter the system due to worn seals and components. Over time, as the pistons move back and forth, the seals will break down, allowing air into the system. When worn or broken components are changed, pockets of air may also move into the system and be difficult to purge out. The traditional bleeding method of having one person depress the brake pedal while another person bleeds the air at the wheel can be very time-consuming and often is not the most effective for removing 100 percent of the air. Vacuum and pres- sure systems offer better options for removing the air. Moisture By design, a brake fluid is formulated to absorb moisture. Otherwise, water mole- cules could rot the internal components and damage the braking system. Of course, this property comes at a price. As the brake fluid absorbs moisture, it lessens the fluid's performance. The high tempera- tures common with braking systems can result in this moisture vaporizing, which causes the fluid to become compressible and gives you that "spongy" feeling. Not all brake fluids have this property of absorbing moisture. Silicone fluids will only absorb so much moisture, leaving the rest to stay in free form and sink to low spots in the system. This can lead to corrosion. So whether the contamination is from air, water, temperature or foreign materials, your brake fluid will need to be changed. I recently went through this with my car. While at the quarter-mile drag strip on a sunny day, I lost the functionality of my clutch. With a combination of hard launches and old fluid in my reservoir, my clutch pedal fell dead to the floor as I was trying to shift into third gear. After giving the car some time to cool down, I discov- ered that my clutch pedal came back to its normal firmness. I sought a solution to this problem and found I was not the only one to experience this phenomenon. I set out to correct the issue by purchasing some components that didn't allow the supply line to the slave cylinder to be exposed to such intense heat. Another improvement was adding a remote bleeder to the system. With my new hard- ware and fluid, I'm now able to change out the fluid quickly without much hassle and haven't had a disappearing pedal since. Classifications and Standards Brake fluids are categorized into four main classifications by the U.S. Depart- ment of Transportation (DOT): DOT 3, DOT 4, DOT 5 and DOT 5.1. Most fluids fall into the DOT 3, DOT 4 or DOT 5.1 clas- sification. These fluids are all hygroscopic, which means they absorb moisture from the air. DOT 5 fluids are not hygroscopic but are often used in vehicles that sit for long periods of time, such as collector cars or military vehicles. The chemical composition of the fluids also changes with the different classifica- tions. DOT 3 fluids are glycol ether based. DOT 4 fluids are a mixture of glycol ether with borate ester. DOT 5.1 fluids use borate ester with glycol ether blended in, while DOT 5 fluids are silicone based. The Federal Motor Vehicle Safety Stan- dards (FMVSS) No. 116 defines the properties that a brake fluid must have to be catego- rized into one of the DOT classifications. The table below shows some of the limits a fluid must meet to fit within this classification. These properties affect how a brake fluid performs. Boiling point is one of the major indicators of how the braking or clutch system will react. During braking, wheel cylinders and brake calipers are subjected to very high temperatures due to the friction from the brake pads coming into contact with the drum or disk. During road course events and track days, it's not www.machinerylubrication.com | September - October 2016 | 51 Brake fluid is no different than the other fluids in your vehicle and should be replaced. DRY BOILING POINT WET BOILING POINT VISCOSITY @ -40°C VISCOSITY @ 100°C CHEMICAL COMPOSITION DOT 2 190°C/374°F 140°C/284°F - - Castor Oil/ Alcohol DOT 3 205°C/401°F 140°C/284°F Max. 1,500 mm²/s Min. 1.5 mm²/s Glycol Ether DOT 4 230°C/446°F 155°C/311°F Max. 1,800 mm²/s Min. 1.5 mm²/s Glycol Ether/ Borate Ester DOT 4+, SUPER DOT 4 300°C/572°F 180°C/356°F Max. 750 mm²/s Min. 1.5 mm²/s Glycol Ether/ Borate Ester DOT 5 260°C/500°F 180°C/356°F Max. 900 mm²/s Min. 1.5 mm²/s Silicone DOT 5.1 260°C/500°F 180°C/356°F Max. 900 mm²/s Min. 1.5 mm²/s Borate Ester/ Glycol Ether LHM+ 249°C/480°F 249°C/480°F 1,000-1,200 mm²/s 6-6.5 mm²/s Mineral Oil DOT 4+ and Super DOT 4 fluids are not governed by FMVSS No. 116. The values shown are typical of DOT 4+ and Super DOT 4 fluids on the market. Most meet or exceed DOT 5.1 specifications for boiling points.