This article is from a April 2004 issue of:
Undercar Digest, published by M D Publications, Inc.
The first step in a brake inspection - often overlooked - is to note whether the red brake warning light and the amber anti-lock light illuminate during the key-on, engine-crank cycle. If either one does not, you must treat it as though it stayed on after engine startup. On this vehicle the amber anti-lock light did not illuminate. Although this article does not address ABS diagnosis, the recommendation to the customer after this brake inspection would be to have the failure of the ABS to self-test diagnosed. This diagnosis, of course, is a procedure you would charge for.
Are the brake lights working? If not, why not? Is it something as simple as a fuse? If so, why did the fuse blow? Replacing a fuse without finding out why it blew is courting trouble.
A technician wipes the outside of the master cylinder before removing the cover to prevent contamination of the fluid.
Carefully inspect the rubber grommet or liner of the cover for any signs of contamination. Petroleum-based or other types of contaminants frequently will cause the liner to be distorted or dimpled. Even though the fluid in the master cylinder may be clean now, the system may have been contaminated previously and not flushed and refilled properly.
You must remove all four wheels for a proper brake inspection. Before removing the front wheels from this S10 pickup, the technician checked for wheel-bearing looseness, which can be mistaken for unusual pad wear or some other brake problem.
Indexing the wheel to a stud will assure that you return the wheel to the same position, preventing pull or vibration problems caused by changing the mounting position of a tire.
If the wheel bearings are loose, remove the wheel, remove the hub dust cap and adjust the bearings.
Even though you will use a micrometer to measure rotor thickness and compare it with specifications, it's not a bad idea to run your fingernail across the surface of a rotor, feeling for any unusual wear patterns, material-transfer buildup or other problems.
In addition to visually inspecting the rotor and measuring it with a micrometer, measure runout. Mark the rotor in one spot, set up a dial indicator and rotate the rotor one full revolution, noting the amount of runout. Even if you don't observe pedal pulsation when driving the vehicle, if runout is beyond specifications, it is reason enough to recommend rotor service such as machining.
In addition to visually checking thickness of friction material on a brake pad, there are tools and gauges you can place between the rotor and metal backing to measure thickness. The new owner of this used pickup, which supposedly had had a recent brake job, wanted to know whether the former owner had told the truth. On the basis of lining thickness and other considerations, it appeared that the brake job was fresh, even though it wasn't necessarily as thorough as it should have been.
Note how the pads index or seat into the lower part of the caliper. If they don't seat correctly, there may be a clicking or clunking noise on initial brake application. On some brake systems you must cinch down the pads on the caliper to assure that they are tight. Also check for excessive rust or scale buildup between the pad and the caliper mount.
This is a controversial part of a brake inspection. I believe that operation of a caliper that does not slide in and out freely with hand force will be restricted. In addition, the force required to move the caliper must be equal side to side. The right-front caliper being tested here did not slide with normal force and appeared to be restricted compared with the left-front caliper. You must determine the reason as part of the brake inspection.
The caliper mounting pin appears to be amply lubricated, but the presence of lubricant doesn't exclude the possibility of other problems.
Before removing calipers, install a line lock on the flexible brake hose and open the bleeder valve. This prevents fluid from being forced upstream, which can cause serious problems. After removing the caliper, if you have to retract the piston or force the caliper in and out for the pads to clear a rust ridge on the rotor, you won't have fluid being forced upstream and possibly causing secondary problems.
The caliper is being removed for inspection purposes. If you skip this step, you might miss the real reason for a problem such as the restricted caliper movement on this vehicle.
When removed from the vehicle, this looked like a typical used GM caliper mounting pin.
After cleaning the pin, the technician discovered that corrosion had eaten through the protective coating on the end and prevented it from sliding correctly. Is it just a problem with the caliper-mounting pin, or are there other related hardware issues?
Look at the wear indicator on the pad. Wear indicators are usually inboard, but on some late-model systems they also can be outboard. Note whether the shoe has a wear indicator and, if it does, whether it is shaped correctly or has been bent out of the way. There have been instances of technicians bending wear indicators away from the rotors to allow for additional pad wear without the noise. How unsafe is it, and what degree of liability do you incur by doing so?
Check for freedom of movement of the metal sleeve that fits into the ear of the caliper and is retained or positioned by rubber O-rings inside the caliper.
Push it in each direction. If it doesn't move freely or if you can't move it with hand force, determine the reason. On a system that is worn and has never been serviced, you may have to use channel locks or another type of assisting tool to remove the sleeve. The O-rings inside the groove in the caliper are replaceable items, as are the sleeves. Unfortunately, many people do not include them in the price of a brake job even though they should be replaced to ensure proper caliper operation.
After disassembling a brake system for inspection, do you just put it back together or do you lubricate some of the obvious areas? The technician performing this inspection chose to lubricate the back of the brake pad where it comes in contact with the piston.
He also lubricated the caliper-mounting pins. Some shops just put parts back together dry. Whether to lubricate is a matter of personal preference, but common sense should dictate what to do at this point.
Any metal-to-metal contact points not only should be lubricated but also should be checked for unusual wear or fretting caused by lack of lubrication. Although this brake system doesn't have wear points that are subject to that type of problem, you still should check for unusual wear.
Look for fluid seepage where this flexible brake hose connects to the caliper. Sometimes when technicians replace calipers, they forget about the old washer on the hose and add a new one, and sometimes these lines are not torqued or tightened correctly. There are also other reasons for seepage.
When removing a rear brake drum, rapping on the surface of the drum with a brass or other type of hammer is common practice. The hammer blow loosens rust and scale that have built up in the drum, which usually then will pop off. After removing the drum, look inside for evidence of friction material worn from the brake linings. If the vehicle has enough miles on it since the last brake job that there should be significant wear but you see no evidence of wear, determine why.
If one side shows evidence of wear and the other doesn't, it's a good bet that something is restricting brake operation on the side with no wear. It could be a frozen wheel cylinder, a restricted hydraulic line or something else. After you remove the drum, physically inspect it and move on to the shoe's upper attachment point.
On this duo-servo brake system, make sure the retainer that goes over the upper mounting pin is sitting flat against both shoes. IF it is partially on the shoulder of the pin, reposition it correctly. This also is a good time to visually inspect the wheel cylinder for external leakage. On this type of system with internal dust boots, fluid can leak and build up behind the boots. Determining whether the cylinder is leaking is rather difficult without removing the brake shoes.
Also look at the top of the shoes. With the brakes released, both shoes should be sitting against the top anchor. If they aren't, the reason could be the parking-brake cable or worn mounting pads on the backing plate.
When a duo-servo drum-brake system is installed correctly, you can grasp it at the bottom near the star wheel, on either the lower part of the shoes as shown or the star wheel itself and, using the top anchor pin as the pivot point, rotate it both directions in an easy, controlled fashion without anything coming off, catching or binding. If you can do this the system is assembled. Don't believe it? Assemble a system incorrectly and try it. You'll find some restriction.
Also inspect the shoes. This secondary shoe has more wear in the center than on either end. Whether this is a problem must be determined by your experience. Lining wear of this type is not unusual, and the amount of difference shown would not raise any red flags.
The primary shoe has a much better wear or contact pattern than the secondary shoe. Again, there are no unusual grooves, scoring or other indications that the primary and secondary shoes are not working together.
If you cannot rotate the star wheel by hand after moving the actuating lever away from it, the star wheel is restricted. Normal self-adjusting mechanisms that actuate when the vehicle backs up exert little force on the star wheel. Those that are applied by parking brakes usually have additional force. Therefore, if you can't rotate the star wheel by hand, the self adjuster probably will not rotate it when the vehicle is backed up and there is ample clearance between the linings and the drums to allow the lever to turn the star wheel a notch.
The last part of a brake inspection, particularly on rear brakes, is to check the parking-brake cables. The technician checks the front cable ahead of where it tees off to the rear wheels. That cable generally is inspected for wear, correct routing and other visually detectable problems.
The technician pulls on the cable for the right-rear brake. When you pull on this cable, the shoe should move away from the anchor. If it doesn't, determine why. After releasing the cable, check to see whether the shoes return to the top anchor as shown in Photo 28. If they don't, the cable is binding or sticking.
This article is from a April 2004 issue of:
Undercar Digest, published by M D Publications, Inc.