Magazine for the auto service professional
Issue link: http://asp.epubxp.com/i/780462
36 | ASP February 2017 Technical driving, silicone fluid is certainly an option to consider. But for your customers' daily driving and especially for performance driving, stick with DOT 3, DOT 4 or DOT 5.1 glycol fluid that offers a high wet boiling point. Brake fluid boiling points As the brake system absorbs airborne mois- ture, this will lower the boiling point of the fluid. e rated dry boiling point represents the point at which fluid will boil when fresh, with no absorbed moisture. e lower or "wet" boiling point represents the expected performance of the brake fluid after it has been in the car for approximately one year. For a routinely driven street vehicle, the wet boiling point is more important than the dry, because the brake fluid is going to be in use for much longer periods. In addition to lower- ing the boiling point, water in the system will eventually corrode the metal surfaces within the system. While all brake fluids feature corrosion inhibitors, these inhibitors eventually break down and become less effective over time. Glycol brake fluids should be completely drained and replaced, on average, every two years or 24,000 miles in order to maintain optimum fluid performance, regardless of how much the vehicle is driven or stored. Silicone brake fluid offers very high dry and wet boiling points. However, silicone tends to be more compressible because it will absorb more air than a glycol based fluid, and will infuse itself with air. e dissolved air results in a slightly spongy pedal feel. Especially from a performance standpoint, it's recommended to always select a brake fluid that's rated with a dry boiling point of at least 450 degrees Fahrenheit. e two most important aspects of a perfor- mance brake fluid are compressibility and its rate of moisture assimilation. Compressibility refers to pedal feel... how firm the pedal feels when the brake pedal is depressed (also assuming the system is properly bled of air). Different brake fluids can dramatically alter pedal feel, with no other system changes. Obviously, a braking system generates fric- tional heat, as the friction material is forced against the surfaces of a rotor disc. is heat is quickly absorbed into the fluid, elevating its temperature. e published "typical" dry and wet boiling points indicate at what temperature the fluid will begin to boil. e dry boiling point represents the point at which fluid will boil when it's fresh and contains no water. e wet boiling point indicates the temperature at which the fluid can begin to boil when it contains as little as 1% to 2% moisture. e older the fluid, the more moisture Examples of performance brake fluid specifications Brand/model Dry boil point Wet boil point Centric STR 600 594 degrees F 404 degrees F Centric STR 660 Ultra 622 degrees F 404 degrees F Brake Man Hi Temp 577 fluid 577 degrees F 300 degrees F Castrol SRF 590 degrees F 518 degrees F Castrol LMA DOT 3/4 4 46 degrees F 311 degrees F ATE Type 200 DOT 4 536 degrees F 388 degrees F (same with amber tint) Ford Heavy Duty DOT 3 550 degrees F 90 degrees F (inexpensive, but must be changed frequently) TBM DOT 5.1 Xtreme 6 612 degrees F 400 degrees F (high rejection to moisture absorption)