ROTORS & DRUMS
When linings are replaced, the rotors and drums may need to be resurfaced or replaced depending on their condition. If a rotor is worn to minimum thickness, or a drum is worn to maximum diameter, replacement is necessary. Most rotors are made of cast iron, but composite rotors have a thin, stamped steel center hat attached to a cast iron rotor ring. Composite rotors are more difficult to resurface and more prone to pedal pulsation problems than one-piece cast rotors. They are also more expensive than cast. Composite rotors can be replaced with cast rotors as long as both rotors are replaced at the same time (don’t intermix different kinds of rotors side to side).

Most front rotors are vented, while most rear rotors are not because more cooling is usually needed for the harder-working front brakes. Most rotors are interchangeable left to right, but some are directional, so pay close attention to the catalog listings when looking up part numbers. New rotors are ready to install and do not require additional resurfacing (turning rotors unnecessarily shortens their service life).

Other parts that wear out over time include hydraulic components such as the calipers, wheel cylinders and master cylinder.

CALIPERS
Most calipers have one or two pistons, but some have up to four pistons mounted in a rigid housing. Most calipers are cast iron (though some are aluminum) with steel or molded phenolic (plastic) pistons. Most calipers are a floating design with slides or bushings that allow the caliper to move sideways and center itself over the rotor when the brakes are applied. Others are a fixed design with rigid mounts and do not move. If the slides or bushings on a floating caliper become badly corroded or worn, it may prevent the caliper from sliding, causing uneven pad wear. The inside pad will wear faster than the outside pad. If a piston in either type of caliper sticks, the caliper may not release, which can cause the brake to drag, rapid pad wear on one side, uneven braking and a pull to one side when the brakes are applied. Leaky piston seals will allow brake fluid to contaminate the brake linings. Leaky calipers must be rebuilt or replaced. Loaded calipers come ready to install with new pads. Bare calipers do not include pads. On vehicles with four-wheel disc brakes, the rear calipers may also include some type of parking brake mechanism. This makes the calipers more complicated and expensive to replace.

WHEEL CYLINDERS
The wheel cylinders inside drum brakes have two opposing pistons that move outward when pressure is applied. The wheel cylinder is mounted on the brake backing plate, and has dust seals over the pistons to keep out dust and water. Each piston has a cup-shaped seal for the fluid inside. Common problems with wheel cylinders include fluid leaks and sticking. Wheel cylinders can be rebuilt or replaced. Leaking fluid can contaminate the brake shoes, requiring their replacement as well.

Wear in the master cylinder may allow fluid to leak past the piston or shaft seals. A symptom of a bad master cylinder is a brake pedal that slowly sinks to the floor when braking at a stop light. Leaks or failure to hold pressure require rebuilding or replacing the master cylinder. Rebuilding aluminum master cylinders is not recommended. On some older vehicles with ABS, the master cylinder is part of the ABS modulator and is very expensive to replace.

Rubber brake hoses can also deteriorate with age and leak. Any hose that is cracked, bulging, leaking or damaged should be replaced without delay because of the danger of brake failure should the hose leak. Steel brake lines can corrode internally or externally. Replacement brake lines must be steel with double-flared or ISO end fittings.

BRAKE FLUID
Brake fluid also wears out over time and should be replaced when the brakes are serviced.

The main issue here is moisture contamination that causes a breakdown of corrosion inhibitors in the fluid and lowers the fluid’s boiling temperature (which increases the risk of fluid boil and pedal fade under hard use). DOT 3 and DOT 4 brake fluid are the two main types and both are glycol-based hydraulic fluid. DOT 5 fluid is a silicone-based fluid and is used only for special applications (like older vehicles that sit for long periods of time or are operated in extremely wet environments). DOT 4 has a higher temperature rating than DOT 3 and is used in many European vehicles. Use the type of fluid specified by the vehicle manufacturer.

Related items that may also need to be replaced when servicing the brakes include the wheel bearings and seals. On older vehicles with serviceable wheel bearings, the grease seals should always be replaced when the bearings are cleaned and repacked with grease. Special high-temperature wheel bearing grease is required (never ordinary chassis grease).

Disc and drum brake hardware should also be replaced when the brakes are serviced. Drum hardware includes the return springs, holddown springs, self-adjusters and other cables, clips or springs used in the brake assembly. Return springs that pull the shoes back away from the drum when the brakes are released may become weak with age, allowing the brakes to drag. Self-adjusters can become corroded and stick, causing increased pedal travel as the shoes wear. On disc brakes, the hardware includes slides and bushings that can become worn and corroded and anti-rattle clips and springs that reduce noise. A high-temperature, moly-based brake grease should be used to lubricate slides, bushings and shoe pads on drum brake backing plates.

The cooling system includes the radiator, radiator cap, coolant reservoir, fan, water pump, thermostat, hoses, belts and antifreeze. Related parts include the coolant sensor and fan relay.

The radiator’s job is to cool off the hot coolant after it leaves the engine block. The radiator is mounted up front so it gets good airflow when the vehicle is moving. But when the vehicle is stopped or traveling at low speed, additional airflow must be provided by a belt-driven or electric fan. The fan also operates when the A/C is on.

The radiators in most late-model vehicles are aluminum with plastic end tanks. Most older vehicles have copper/brass radiators. Both types are vulnerable to internal corrosion caused by coolant neglect. Leaks can sometimes be stopped by adding a sealer product to the coolant, but eventually the radiator will have to be repaired or replaced if it is leaking. Replacement radiators should have the same cooling capacity (or better) as the original and the same hose connections. Cooling capacity is determined by the thickness of the radiator, the number of fins and tubes and/or the design of the fins and tubes. Increased cooling capacity is recommended for towing and performance applications.

Connected to the radiator by a tube or hose is a plastic coolant reservoir. The reservoir typically holds up to a quart of coolant and prevents the loss of coolant if the engine overheats. The coolant level is maintained at the reservoir (never open a hot radiator cap!)

The radiator cap seals the radiator and pressurizes the coolant inside. This raises the temperature at which the coolant normally boils for added boilover protection during hot weather. System pressure ratings vary from five to 15 psi. A weak spring in the cap or a leaky seal can allow coolant to escape from the radiator, which may lead to overheating. Pressure testing the radiator cap will reveal its condition. If the cap can’t hold its rated pressure, it must be replaced with one that has the correct pressure rating for the application.

The water pump is the heart of the cooling system. It circulates coolant between the engine and radiator to keep the engine at normal operating temperature. The pump is belt-driven and consists of an impeller mounted on a shaft inside a cast or stamped steel housing. The pump is usually mounted on the front of the engine. On some overhead cam (OHC) engines, the pump is mounted under the timing belt and requires considerable labor to replace. For this reason, you should recommend replacing the pump if the timing belt is being replaced for scheduled maintenance (recommended every 60,000 to 100,000 miles depending on the application.) The service life of the water pump and timing belt are about the same, so changing both at the same time can save the vehicle owner money on future repairs. Water pumps don’t last forever, and leaks around the pump shaft seal and bearing can quickly lead to overheating. Any water pump that is leaking, making noise or has excessive shaft play should be replaced. Replacement options include remanufactured and new pumps.

The water pump may also be driven by a V-belt or a flat serpentine belt. The same belt may also drive other engine accessories. Belts deteriorate with age and should be replaced if frayed, cracked, glazed or oil-soaked. Replacement belt length and width must be the same as the original. On vehicles with serpentine belts, the automatic tensioner may also need to be replaced if it’s sticking, making noise or cannot maintain proper belt tension. Belt idler pulleys should also be replaced if noisy, worn or sticking.

For temperature control, the cooling system requires a thermostat. It is usually located in a housing where the upper radiator hose connects to the engine. The thermostat does two things: it allows the engine to warm-up quickly (which reduces cold emissions and fuel consumption) and to maintain a consistent operating temperature (also important for low emissions, good fuel economy and performance). The thermostat has a temperature-sensitive valve that remains closed and blocks the flow of coolant until the engine reaches 195 to 210 degrees. It then opens and allows coolant to flow to the radiator. The thermostat continues to cycle open and shut so the engine will run within a certain temperature range. This is very important on late-model vehicles with computer engine controls because engine temperature affects the fuel feedback control loop, emissions, fuel economy and performance.

If the thermostat sticks shut, the engine will overheat. If it fails to close, the engine will be slow to warm-up, and the heater may not put out much heat when the weather turns cold. Fuel economy, emissions and engine wear will also suffer. Under no circumstances should an engine be run without a thermostat. Replacement thermostats should have the same rating as the original. A slightly hotter thermostat may be used during cold weather for increased heater output, but a colder thermostat should not be used on engines with computer controls. Other items that may be needed when changing a thermostat include a new thermostat housing and gasket or sealer.

To improve cooling, a fan is needed to pull air through the radiator when the vehicle is stopped or traveling at low speed. Older, rear-wheel-drive vehicles may have a belt-driven fan with or without a clutch. The clutch allows some slippage and is used to reduce fan noise at high rpm and to improve fuel economy by reducing drag. Excessive slipping in the clutch, however, may reduce airflow and cause the engine to overheat at low speed. Most newer vehicles have one or two electric cooling fans mounted behind the radiator, and a few have hydraulic fans driven by power steering fluid. Electric fans are powered through a relay and controlled by a coolant temperature switch or the engine computer. A failure of the fan motor, fan relay, coolant temperature switch or a wiring problem that prevents the fan from coming on can cause the engine to overheat.

The major hoses in the cooling system include the upper and lower radiator hoses (the lower one usually attaches to the water pump, and the upper usually attaches to the thermostat housing), plus a pair of heater hoses (one inlet and one outlet) and various connecting hoses and bypass hoses depending on the application. Original equipment hoses are usually molded (preformed) to fit a particular vehicle. Aftermarket replacement hose may be molded (which sometimes requires cutting to length), or straight or ribbed (flexible) to fit a wider variety of applications. Replacement hoses must be the same length and diameter as the original. New clamps should also be installed when hoses are replaced. Hoses should be replaced if leaking, cracked or bulging. Electro chemical degradation (ECD) due to coolant neglect can cause hoses to fail from the inside out.

The coolant that carries heat from the engine to the radiator is a mixture of ethylene glycol antifreeze and water (typically a 50/50 mix.) This combination provides freezing protection down to -34 degrees F and boilover protection up to 265 degrees F with a 15 psi radiator cap. The condition of the coolant is just as important as the strength because corrosion can attack the system from within if the coolant is neglected. The recommended replacement interval for traditional green antifreeze is two years or 30,000 miles. For the newer long-life coolants, the change interval can be as long as five years or 150,000 miles. Most long-life coolants use organic acid technology (OAT) additives that are different from those used in standard antifreeze. Long-life coolants may contain special dyes to distinguish them from ordinary antifreeze. General Motor’s Dex-Cool is dyed orange, for example. Different types of antifreeze should not be intermixed as doing so may reduce the ability of long-life products from protecting the cooling system from corrosion.

The drivetrain consists of everything between the transmission and wheels. On rear-wheel-drive (RWD) cars and trucks, this includes the driveshaft, universal joints, differential and axles. On front-wheel-drive (FWD) cars and minivans it includes the halfshafts, constant velocity joints (CV) and hub assemblies, and on all-wheel-drive (AWD) cars and SUVs and four-wheel-drive (4WD) trucks, it includes a transfer case.

The most often replaced drivetrain components on RWD vehicles are the U-joints. On FWD vehicles, it’s the CV joints. Both are components that eventually wear out.

U-joints are mounted on both ends of the driveshaft on RWD vehicles so the driveline can move up and down to follow the motions of the suspension. Each U-joint consists of a four-point center cross with needle bearing cups mounted in a pair of yokes. Vehicles with two-piece driveshafts have additional U-joints and/or a CV joint where the two shafts are joined and supported by a center carrier bearing.

U-joints are "phased" (positioned) 90 degrees to one another to reduce driveline vibrations. Almost all late-model OE U-joints are sealed, but some older vehicles have grease fittings for extended life. Some aftermarket replacement U-joints are also avail