Auto Service Professional

APR 2018

Magazine for the auto service professional

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44 | ASP April 2018 Technical and slowly releasing several times until all traces of air bubbles are eliminated. In some cases, even after bench bleeding via the output ports, it may be necessary to also bleed via the master cylinder's bleed ports (if so equipped). Once all trapped air has been released, be sure to keep the fluid level in the reservoirs sufficiently high to avoid expos- ing the outlet orifices at the bottom of the reservoir wells before installing the master cylinder. Naturally, during system bleeding, constantly keep an eye on the reservoir fluid level to avoid running the master dry. A note regarding the use of DOT 5 sili- cone brake fluid: Silicone fluid won't absorb moisture, and it won't damage a painted surface. Many customers choose this type of fluid because they assume that, since it won't absorb moisture from the air, this will prevent brake line/system corrosion. However, silicone fluid can still carry moisture on its surface, so while it may help to reduce the chance of corrosion, it isn't a magic potion. e only real advantage of using silicone fluid is because it won't eat into a painted surface. During a fill/top-off, or if the system leaks, it may make a slimy mess, but it won't lift paint from nearby areas. From a functional and performance aspect, it's simply not a good choice. Silicone fluid can become very unstable at temperature, resulting in a low or ever-changing pedal feel. It's also affected by atmospheric pressure. At higher temperatures, it expands, requiring more pedal pressure and more pedal travel. If the brakes are applied rapidly (quickly pumping the pedal manually during a panic stop), the fluid can tend to aerate and foam, resulting in a drop of pressure. For this same reason, NEVER use silicone fluid in a vehicle that's equipped with ABS. When ABS activates, fluid pressure is modulated quickly, which can easily result in the foaming issue. Pedal pulsation A variety of conditions can cause a pedal pulsation or vibration problem that can be traced to rotor runout and/or excessive rotor Heavy rust or debris buildup inside a vented rotor can severely reduce heat dissipation and can lead to overheated and distorted/warped rotor discs. Even if the disc surface is or can be machined, if the vanes cannot be cleaned out, the rotor should be replaced.

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