Auto Service Professional

APR 2018

Magazine for the auto service professional

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42 | ASP April 2018 Technical Brake bleeding Air inside the hydraulic system causes a mushy pedal (or loss of pedal, depending on the amount of air in the system). Remember, you can compress air, but you can't compress fluid. A regular schedule of bleeding provides two benefits: it pro- vides an opportunity to remove any air that has been sucked into the system during use, and it gives you a perfect chance to freshen the brake fluid. Glycol brake fluid is hygroscopic, which means that it sucks up water like a magnet grabs grinding dust from your workbench. As little as a 3% moisture content can cut the brake fluid's boiling point in half! Brake fluid, even the best and most expensive fluid, will NOT last forever. is is a consum- able product, and should be changed on a routine basis. Vehicles equipped with an anti-lock braking system (ABS) require special attention when bleeding the system. If you attempt to flush and refill in the "normal" manner, you may end up with a soft brake pedal. Why? Because you haven't flushed the separate passages in the module for solenoid activation. You may need to use a scan tool to cycle the system. Always refer to the service manual for specific brake system bleeding steps. Improper bleeding procedures can also cause damage to a previously good master cylinder. If a master cylinder has been operat- ing correctly for many miles of use, and the system is then bled in the course of a normal brake job, the master cylinder's piston seals can be damaged if the pedal is pushed fully to the floor during bleeding. Over the course of long-term use, the piston inside the master cylinder travels a relatively short distance. If the system was poorly maintained and the brake fluid has absorbed excess moisture, the end of the piston bore may feature corrosion build-up in the end of the bore that normally never experiences piston travel. If, during the bleeding process, the pedal is pushed all the way to the floor, the piston is now pushed into the corroded area, possibly damaging the seal. To avoid this during a bleeding job, you can place an obstacle, such as a block of wood, under the pedal to avoid over-traveling the piston. Whenever servicing a vehicle's brake system, examine the fluid in the reservoir. If it appears dirty, vacuum the fluid out of the reservoirs and replace with fresh fluid prior to Rather than attempting to rebuild worn/failed/sticking calipers, replace with new or remanufactured. Often loaded calipers are available that include pads, but depending on the vehicle ap- plication, severe duty pads may be required in the case of towing, heavy-duty or emergency vehicle applications. All photos by Mike Mavrigian

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