Tool Q&A: Brake Service

In this issue, we’ll discuss tools and equipment related to servicing brake systems. Like most service areas, there have been a lot of changes and they just keep on coming.

Q. We plan to purchase a brake lathe sometime this year and would like to know if there are any guidelines we need to follow to ensure the right finish for rotors and drums?

A. Yes, making sure rotors and drums are finished properly can be one of the biggest steps you can take against brake squeal. Before machining rotors and drums, make sure the lathe's cutting bits are sharp before each use. Check the lathe's cutting depth, speed and feed for proper setting. You can achieve an acceptable, non-directional surface for rotors by using the following settings:

  • Spindle speed for the first and last cuts: 150 rpm.
  • Depth for the first cut: 0.005" - 0.010".
  • Depth for the last cut: 0.004".
  • Cross feed per rpm for the first cut: 0.006" - 0.010".
  • Cross feed per rpm for the last cut: 0.002" maximum.

Use a dampening device during machining to prevent a chatter-related rough finish. After machining, use a lathe swirl grinder or sand in a circular motion with a piece of #120 emery cloth. A word of caution: Remember that there are operating differences between the various manufacturers’ brake lathes. Accordingly, the lathe manufacturer’s recommendations should be heeded at all times. Should the spindle speed (called drive speed with on-the-car resurfacers) or crossfeed speed be excessive, you can wind up “threading” the rotor like a phonograph record. A feed rate that’s too slow can also cause problems. For instance, too slow a crossfeed can result in a rubbing of the surface, resulting in an unsatisfactory finish.

Q. How do we best prep rotors for installation on the car?

A. If you’ve refinished rotors, ensure a non-directional finish with #120 to #150 emery cloth/sandpaper or use a hone specifically designed for this purpose. When finished, wash the rotors with mild soap and water. Prior to installation, ensure that the rotor-to-hub mounting surfaces are clean and rust free. Use a wire brush or small angle grinder as needed until these surfaces are pristine. Failing to do this last, but critical, step can result in rotor runout and a comeback.

Q. We have an ongoing debate in our shop about the replacement of brake fluid. Some of our techs believe fluid ought to be replaced whenever it starts to look dirty. Other techs believe the fluid should only be replaced during brake work. What's the best policy?

A. For starters, it all comes back to the manufacturer's recommendations. Some manufacturers are more outspoken on this maintenance topic than others. Some may require a complete fluid change every two years while others may not say much at all. To help eliminate the guesswork, you may want to consider one of the brake fluid testers on the market. For example, brake fluid test strips indicate fluid condition by dipping them into the fluid and then comparing the color on the strip to a condition chart. There are also electronic testers that you can use to probe the fluid and take fluid condition readings. Remember, make sure to replace fluid with the proper type; use DOT3 or DOT4 as recommended.

Q. What's the best method of cleaning off brake parts during inspection and service?

A. Nowadays, a brake washer using aqueous-based solvent is the best way to go to reduce exposure to brake dust. Unlike the old days, where air pressure sent brake dust everywhere, a brake washer takes the residue away safely and effectively. Brake washers come in a variety of sizes, ranging from large units intended for production work, all the way down to small, portable units. No matter which unit you choose, make the proper use of a brake washer an integral part of brake service.

Q. We’re unsure about the procedure for checking rotors. Some of my guys believe that all you need to do is inspect the rotor surface for excessive rust, gouges and other surface problems. Is there a more conclusive way to do this?

A. Yes, rotor inspection should never be a guesswork proposition. First of all, start with a rotor thickness check so you can determine whether the rotor is even in the serviceable thickness range. You can perform this using a rotor micrometer, which has one flat contact point and a pointed contact point on the other side. Take measurements at eight different locations throughout one complete rotation of the rotor. If the rotor has any grooves, make sure the pointed contact point of the micrometer touches in the groove, not on the shoulder. After you take your series of measurements, compare them to specifications. If the rotor measures at or below its specified discard thickness, replace the rotor. If your measurements show thickness variations outside the manufacturer’s specifications—called parallelism—this will also call for rotor replacement.

Q. Is it better to use a rotor micrometer or a dial indicator for determining rotor parallelism?

A. The two tools don’t really match up “eye-to-eye,” because they’re used differently. A micrometer is the tool to use to check for parallelism, but a dial indicator is the tool to use to determine whether excessive lateral runout exists. Lateral runout occurs when a rotor spins off its axis. Excessive parallelism and lateral runout can both cause a brake pedal pulsation, but for different reasons. That’s why you should have both tools in your brake diagnostic arsenal in order to properly diagnose a pedal pulsation problem.

Q. We need to service the rear brakes on VWs and Audis with electronic parking brake systems. Is there a special tool for these or do you use a standard caliper piston retracting tool?

A. Don’t force the caliper pistons back in using conventional methods on these cars or you will damage the calipers! To properly retract the caliper pistons, use either an appropriate scan tool or dedicated electronic retraction tool specifically made for this purpose.

Q. Brake fluid flushing seems to be more critical, the more I read. What should I consider if I plan to take on this service?

A. First of all, shop around for a quality pressure bleeding system that’s easy to use and fits all the vehicles you service. If you can, try before you buy because some aspects of using a tool just never appear until you’ve actually had some hands-on time with the tool. Next, consider the scan tools you may have and their capabilities. Proper flushing/bleeding of a system with antilock brakes (ABS) may require interacting with the brake hydraulic system through the system pump and solenoids. Always check service information to be sure of what the procedure requires for each make and model. Jumping into a system flush or bleed without knowing what you’re up against could cause you some major headaches. At the very least, you may not be able to get all the old fluid out of the system. Even worse, you could introduce a problem into the hydraulic system, causing a spongy pedal that’s difficult to get rid of. If you have any doubts whatsoever about performing the procedure due to a lack of equipment or information, steer clear. Proceed only when you know you have all the right stuff.

Q. What do I need in order to diagnose ABS systems?

A. A multi-featured scan tool is an excellent starting place for ABS diagnostics. To be effective, your scan tool needs to perform three essential functions: 1) retrieve ABS trouble codes, 2) display ABS diagnostic data, and 3) perform various system function tests. Some scan tools also have the capability of capturing and displaying ABS “freeze frame” data, which is the data that transpired at the time an ABS trouble code registered. This data can be used to help reveal more specific details when pinpointing the cause of an ABS code. From there, a digital multimeter (DMM) can be a helpful diagnostic ally, helping you to perform pinpoint tests that lead to the root of the ABS problem. For example, let’s say an ABS trouble code clues you towards a bad wheel speed sensor. Using appropriate diagnostic information, you would then follow the flowcharts and steps to check the sensor and its circuit.

Thanks for checking out this month’s Tool Q&A column. Remember, this is your column, because it’s based off your questions. So, let PTEN know what’s on your mind when it comes to tools and equipment.