Brake basics booster shot
Weber is president of Virginia-based Write Stuff. He is an award-winning freelance automotive and technical writer and photographer with over two decades of journalism experience. He is an ASE-certified Master Automobile Technician, and has worked on automobiles, trucks and small engines. He is a member of the Society of Automotive Engineers (SAE) and numerous other automotive trade associations. He has worked as an auto service technician, a shop manager and a regional manager for an automotive service franchise operation.
When was the last time you had a tetanus shot, asks the doctor when you arrive to have that cut stitched up. You need a booster every 10 years. Every year, 10% to 20% of people who contract tetanus will die. Auto repair is risky business. Make sure your booster is up-to-date.
Occasionally, we could use a booster when it comes to doing routine repairs like brake jobs. Let’s review brake system basics of pads and shoes, rotors and drums, and hydraulics.
Brakes simply turn the kinetic energy of a moving vehicle into heat energy as they stop the vehicle. That heat energy first is transferred to the rotors or drums and then is subsequently dissipated into the air. Fins between the rotor halves or on the outside of drums help with dissipation.
Friction materials have come a long way since the wood or leather block. (See the January/February 2015 issue of Auto Service Professional.)
Today’s brake linings run the gamut from semi-organic to semi-metallic to ceramic to various sophisticated blends.
Motorists want good feel and no noise from their brakes. Long ago, asbestos was the go-to friction material. It provided excellent stopping power, pedal feel and long life while being virtually noiseless.
Today, we have several options.
Non-metallic pads and shoes are typically a combination of synthetic materials that are easy on the rotors and drums, provide good stopping power and pedal feel. The down side is that they wear rather quickly and usually leave lots of dust behind.
Semi-metallic pads and shoes combine the synthetic materials with a measure of powdered or flaked metal. On the plus side, they are long-lasting and resist fade. On the negative side, they cause increased wear of the rotors and drums and require more force to stop the vehicle.
Ceramic brakes were a mixture of porcelain and clay with copper flakes mixed in. However, copper is being phased out due to water pollution issues. On the plus side, ceramics are quiet, offer good stopping, and fade and wear resistance. They also transfer much of their material to the rotors. The down side is that the friction material may appear to be a rotor problem if it is not transferred evenly due to rotor run-out. Before trashing the rotor, try your on-car brake lathe.
When in doubt, use the type of pads and shoes that originally came with the vehicle. That does not mean that you’re forced to use OE brand friction material, only that the same type of pad is installed (semi-met, ceramic, etc.).
When replacing disc brake pads, do not push the pistons back into the caliper bores without first opening the bleeder valves. Unless they are opened, any crud in the caliper will get pushed back into the brakes lines. By opening the bleeders, some of the crud will be expelled.
Most motorists ignore their brakes until they get some kind of warning, whether that involves inadequate braking, uneven braking or squeal or grinding noise. Inspect the brakes routinely when the customer brings the vehicle in for service. If the linings are getting thin, make a notation on the customer’s receipt to plant the seed for service soon.
Warnings may come in the form or noise, such as the metal tabs the GM has used for years to scrape the rotor when the linings get thin. Of course, vehicles that are not equipped with these sensors will make noise when metal contacts metal. At this point, new rotors are probably in order.
Some vehicles have electrical warnings that illuminate an icon on the instrument panel when an embedded sensor on the pad makes contact with the rotor completing the ground. When replacing pads, make sure that the new pads feature these sensors.
As to a visual inspection, new pads or shoes are called for when the gap between the backing hardware and the rotor or drum surface is thin and nearly gone.
Rotors and drums
Even if the owner has not allowed the pads to wear completely down, his rotors can be damaged by uneven lug nut torque. It can happen when he changes a flat. It can happen when new tires are installed by a careless technician who does not use a torque wrench or, at least, a torque stick.
Unless rotor warp is extreme, it won’t be visible to the naked eye. Use a micrometer to check thickness and thickness variation, and a dial indicator to check run-out. One such condition is a tapered rotor. This is when the inner part of the swept area is thinner than the outer part. Examine the pads for clues of tapering. If new pads are installed on a dished rotor, the pedal will be spongy.
When checking the condition of the rotors, be sure to use all of the lug nuts to hold them in place, not just a few. It can affect your reading. Always refer to the automaker’s run-out tolerance specifications, but in general, run-out should be less than 0.002-inches.
If the rotor is not secured firmly, wobble from uneven clamping might cause a false reading.
You may be lucky enough to true up the rotors with an on-the-car brake lathe. However, today’s rotors are so thin that any machining may put them over the discard limit. If you have to replace one, it is good insurance against a comeback to replace both rotors. The Motorist Assurance Program (MAP) states that when replacing or reconditioning a rotor you may suggest reconditioning or replacing the other rotor on the same axle to eliminate uneven braking behavior.
If you discover lateral run-out beyond 0.002 inches, try repositioning the rotor a couple times. If that does not solve the problem, the best solution may be a shim placed between the rotor and hub. Install the rotor to the hub and secure (if the rotor is independent of the hub, you’ll need to temporarily install all lug nuts to ensure a flat mating against the hub). Tighten lug nuts evenly in a crisscross pattern to a value very close to the specified wheel nut torque value. For example, if the spec calls for 90 ft.-lbs., tightening to 75 ft.-lbs. should be sufficient. Use a torque wrench by hand. Avoid the use of a power tool.
Mount a dial indicator base to a solid, stationary location that won’t allow the gauge to move/wiggle, and position the indicator plunger onto the rotor friction surface, about one inch inboard of the disc outer edge. Make sure that the indicator plunger is 90 degrees to the disc surface. Preload the dial indicator by about 0.050-inch, and then zero the indicator gauge. Slowly rotate the rotor by hand and watch your dial indicator for the high and low spots. Note how much run-out is indicated and mark the high spot. Select a shim that will eliminate the run-out.
Keep in mind that a new rotor may not solve the run-out problem. The hub may be the issue so recheck the run-out when you install a new rotor just in case.
It should go without saying that you should inspect the rotors for physical problems such as cracks, deep scoring, hot spots and excessive rust buildup.
Before installing new rotors, clean them thoroughly remove any grease, oil or metal particles. Aerosol products are fine, but better yet, wash the rotors with soap and hot water. Dawn dish washing liquid is often used to clean waterfowl after an oil spill. If it can clean ducks, geese and swans, it can clean Dodges, GMCs and Subarus.
If you are able to reuse the old rotors, clean them up. Knock off all of the loose dirt, rust and scale. Loose rust scale at the outer edge can break loose, lodging between the pads and rotors. This not only makes an awful noise, but can score both the rotor and pad(s).
While you should always replace caliper and pad hardware, if you decide to salvage the original hardware, at least make sure that all pieces are clean. Caliper sliding pins must be removed, cleaned and re-lubed. Prior to installing the caliper, the piston(s) must be pushed back into their bores to provide clearance for the new and thicker pads. Use a dedicated caliper piston compressor tool to smoothly push the piston(s). If the caliper piston(s) did not move smoothly when pushed back into their bores, replace the caliper. Considering the time required to rebuild a caliper, it’s quicker and usually cheaper (labor-wise) to simply replace the caliper with either a new or reman unit.
According to the MAP: “You are not required to replace or rebuild calipers in axle sets. However, when replacing or rebuilding a caliper... you may suggest servicing, rebuilding, or replacement of the other caliper (on the same axle) for improved performance and preventive maintenance (for example, the part is close to the end of its useful life, replacing the caliper may extend pad life, or contribute to more balanced braking). When installing loaded calipers, it is required that friction material be matched in axle sets for consistent braking characteristics.”
Brake fluid eventually wears out and becomes contaminated by moisture absorbed from the atmosphere, which also lowers the fluid’s boiling point. The corrosion inhibitors’ reserve alkalinity degrades leaving the system vulnerable to corrosion. That can pit the bores of the master cylinder, calipers, wheel cylinders, and ABS components.
Some carmakers, particularly European manufacturers, recommend flushing the brake system every two years or 24,000 miles. If your customer’s owner’s manual does not prescribe routine brake fluid flushing, check the condition of the fluid using test strips.
The test strips indicate the amount of copper in the fluid. Most brake lines are steel, but the interior may be coated with copper. As the reserve corrosion inhibitors in the fluid drop, copper enters the fluid. Before long, corrosion can develop in the caliper bores and wheel cylinders. Even worse, it can occur in the ABS components leading to very expensive repairs. Test strips reveal high levels of copper in the fluid.
The brake lines may develop leaks when the corrosion inhibitors are depleted —especially in the Snow Belt or near the coast from salt spray. Use only double wall steel brake lines that meet Department of Transportation (DOT), Society of Automotive Engineers (SAE), and Federal Motor Vehicle Safety Standards. A leak anywhere means the entire line is compromised.
Inspect the lines for corrosion that could become a potential leak. Pay special attention to areas where the lines are clamped or where water could collect. Check the brake hoses for cracks and dry rot. If one of the brakes drags or will not release, the inner wall of the hose may have separated and imploded, creating, in essence, an internal check valve that won’t allow fluid to return to the master cylinder. A good clue to this problem is a squirt of fluid from the bleeder despite the brakes not being applied. If one hose has age-failed, chances are good that the hose on the opposite side of the axle is in the same condition. It’s best to replace both.
Incidentally, if you are trying to isolate the cause of brake pull, never clamp those hoses with vise grips as you can damage the interior wall of the hose. There are safe hose “hemostats” for this job.
Never splice brake lines using compression fittings. Pre-bent lines are readily available for nearly all vehicles. They beat bending your own by saving time and aggravation. Most older vehicle applications are readily available from specialty brake line sources. These specialty manufacturers usually offer a choice of either steel or stainless steel. Stainless tubing is a popular choice for restorations, and while more costly than steel, the price difference can be surprisingly affordable.
While you are under the car inspecting the brake lines, inspect the parking brake cables and equalizer. Do the cables move freely? Many people who have cars with automatic transmissions fail to use their parking brakes regularly. This can lead to corrosion and frozen cables. If you cannot clean and lube them, replace the cables.
Bleeding and refilling the brake system is not enough. It can, in fact, be detrimental as sludge could be displaced from places such as the bottom of the caliper bore and lodge somewhere else. Machine flushing is the quickest and most effective method. It also minimizes the potential to get air in the system.
Consult the vehicle maker’s specifications for the vehicle’s brake fluid type and service recommendation. This is most often DOT 3 and occasionally DOT 4. But some European vehicles use mineral oil, so pay attention to specifications to avoid cross contamination. Adding mineral oil in a system that requires glycol based fluid will likely ruin the rubber components.
Glycol brake fluid absorbs moisture and can damage paint. Use caution when handling brake fluid. Silicone brake fluid is not recommended due to the potential for aeration if the brake pedal is pumped/pulsed quickly. However, silicone fluid is often popular among restoration fanatics simply because if spilled, it won’t damage a painted surface.
Vacuum operated brake boosters have been around for decades and are seldom a source of trouble. Low vacuum due to a leak is the most common problem. On old cars, or cars fitted with high-performance engines, low vacuum can be an issue. You should read at least 16 inches of vacuum on your gauge. If not, you may want to consider adding an auxiliary vacuum pump.
Hissing from the booster housing can come from an internal leak. If the customer complains of a hard pedal when first starting out, there could be a leak or the check valve could be faulty.
If the vehicle lacks brake boost in freezing temperatures, but eventually returns to normal after warm-up, suspect an ice plug in the vacuum line. Re-route the line to prevent a low spot trap for water.
But wait, there’s more
When you plan to re-install the original brake rotors, it’s always a good idea to place matchmarks before removing the rotor, so that the rotor can be reinstalled in the same location relative to the hub. Any problems with the hub or build-ups of rust can be amplified if the rotor is reinstalled in the wrong clock position.
Proper wheel fastener torque value is essential. Follow the manufacturer’s torque specifications and tightening sequence. That is typically the star pattern on five-lug wheels and crisscross on four- and six-lug wheels. Make sure that all male and female threads are clean and free of burrs, but do not lubricate threads unless specified by the vehicle manufacturer as this can lead to over-tightening. Avoid the temptation to use an impact tool, but if you insist on using one, use torque sticks of the proper rating. Ideally, you should always use a calibrated torque wrench. Over-tightening and uneven tightening are prime causes of rotor warpage and resulting pedal oscillation after a brake job.
Finally, take the vehicle for a drive and burnish the new brakes. It is important to transfer some friction material to the rotors or drums for best braking performance. If you neglect this step, your customer may complain that the new brakes do not work as well as the old ones. Some pad makers claim that burnishing or “pad bedding” is no longer necessary, but it never hurts.
Two methods are acceptable. The first method is to make at least 20 complete stops from 20 mph allowing the brakes to cool for 30 seconds between stops. The second method is to make 20 applications bringing the car from 50 mph to 20 mph using moderate pedal pressure. Again, allow the brakes to cool between slow-downs.
Do not make hard stops with new pads, as you could damage the brake linings due to glazing. Depending on the specific pads and vehicle application, even after burnishing, it may take over 100 more stops to realize peak braking performance.
High quality, professional brake jobs are the prescription for boosting your bottom line. ●