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Tech Tip: Disc Brake Caliper Service

January 31, 2008

By Gary Goms
Import Specialist Contributor
ImportCar Magazine

The repair or replacement of disc brake calipers has always been a controversial topic for most import shop owners and brake technicians. Some recommend replacing the brake calipers along with the brake pads, while others won’t replace the calipers unless they’re found to be defective. For many, routine caliper replacement is a cost issue and many shops deal with this issue by installing remanufactured calipers “loaded” with new brake pads.

But, while a loaded caliper is a very effective method of packaging a product, a few shops prefer buying the caliper separately so that a specific brand of brake pads can be used for specific applications. Whatever the case, the reliability and longevity of the brake service are increased when a new or remanufactured caliper is installed.

A SIMPLE DEVICE
A disc brake caliper has four basic parts: the caliper housing, caliper piston, piston O-ring and piston dust boot. Although the relatively inexpensive single-piston, floating caliper designs are used in most applications, some high-end or high-performance vehicle applications might use a multiple-piston, fixed-caliper design to allow the use of larger rotors and longer pads for better high-speed braking.

The square-cut O-ring is designed to prevent brake pad drag by retracting the caliper piston a few thousandths of an inch when the brake pedal is released. The design of the O-ring is critical because, if the piston doesn’t retract, the brake pads will create unnecessary rolling friction by dragging against the rotor. In addition, the O-ring must hold the piston securely in place to prevent excessive clearance from developing between the brake pad and rotor. In cases where excess pad-to-rotor clearance develops, a loose wheel bearing or excessive rotor runout is responsible for forcing the piston too far into the caliper bore.

The dust boot serves the critical task of preventing water and dirt from entering the caliper piston bore. Most dust boot failures are caused by excessive heat, mechanical damage from road debris or mishandling of the caliper during a brake pad replacement. If the dust boot is damaged, the intrusion of water and dirt will tend to corrode the caliper piston and housing bore, which causes the piston to stick in an extended position.

BRAKE CALIPER INSPECTION
In warm, dry climates, brake calipers often last the life of the vehicle. However, when calipers are exposed to high humidity and road salt, their service lives are drastically shortened. Before removing the caliper from the vehicle, always loosen the caliper bleed screw to ensure that the bleed screw isn’t seized to the caliper. When compressing the piston into the caliper with a C-clamp or brake pad spreader, the bleed screw should be opened to prevent flushing sludge into the anti-lock brake and master cylinder assemblies and also to allow sludge to be evacuated from the caliper. If the caliper piston can’t be seated sufficiently to allow the caliper to be fitted over the pads and rotor, the caliper might contain excessive sludge.

Clamping a brake hose shut to prevent back flushing debris into the hydraulic system remains a highly controversial topic. Some technicians feel that clamping the hose shut might damage the hose internally and lead to a subsequent failure. Other technicians feel that clamping the brake hose shut is permissible if the hose appears pliable and the clamping is done with a set of smooth-jawed pliers designed for this purpose. On the other hand, if the hose is cracked or hardened from the elements, the hose should be replaced. Current expert opinion indicates that clamping brake hoses presents more dangers than benefits and should be avoided when possible.

Last, if the piston is difficult to seat into the caliper or if the caliper boot is ruptured or appears weather hardened, the caliper should be replaced with a new or remanufactured unit. In most cases, if the bleed screw is broken, the piston is seized to the caliper bore, or the piston seal itself is leaking, it’s usually more economical to replace the caliper with a new or reman-ufactured unit than to attempt to bench rebuild the caliper assembly with a new piston and seal kit.

DIAGNOSTIC SUMMARY
Before removing the wheels from the vehicle, always spin the wheels by hand to see if the brake is dragging. If the wheel doesn’t spin freely, the caliper piston is stuck in its bore. When removing the caliper assemblies from the rotors, it’s important to assess the wear pattern of the brake pads. Excess pad wear at one wheel pad might indicate that the wheel’s caliper isn’t releasing due to a stuck piston or internal blockage in the brake hose. While blockage in a brake hose is a relatively rare failure, it should never be discounted when a brake pull develops or excessive pad wear at one wheel is detected. Brake pads with a crooked wear pattern indicate that the caliper isn’t parallel with the brake rotor. In this case, the caliper mount or the spindle itself might be bent due to collision damage.

Brake pull can be caused by a stuck caliper dragging the brake pads against the rotor. In this case, the pull is caused by heat from the dragging brake pads momentarily changing the coefficient of friction between the pad and rotor. In other cases, brake pull can be caused by poor wheel alignment, loose steering linkage, loose or bent suspension components, mismatched tires, mismatched wheel offsets, under-inflated tires or a defective tire casing.

When diagnosing a brake pull condition, using a non-contact pyrometer to measure relative rotor and caliper temperatures after test-driving the brakes will indicate if a dragging brake is causing the problem. To illustrate, a few minutes of normal braking might produce rotor temperatures of about 200 degrees per side. A significantly higher rotor temperature at one wheel indicates that the brake is dragging. On the other hand, if the temperature of one rotor is significantly colder than the rotors or drums at the other three wheels, the brake might not be working due to a blocked brake hose or similar pressure restriction in the brake line.

Brake squeal is perhaps the most challenging problem to diagnose. Brake noise is essentially caused by the brake pad vibrating against the rotor. Although the composition of the friction material and brake rotors has a great effect on how much noise is generated, it’s important to remember that the brake pad shims and caliper mounting hardware must also be present and in good condition to help dampen high-pitched squealing noises.

CALIPER INSTALLATION
Before installing a new caliper, make sure that the caliper matches the application. The bleeder screw, for example, should point to the top and the caliper piston material and mounting hardware should be the same as original equipment. Before installing the new caliper, the caliper mounting bolts should be checked for correct torque. All bolts originally installed with thread-locking compound should be reinstalled with an equivalent compound. To provide optimum performance, the mount guides and/or guide bolts should be cleaned and lubricated with high-temperature caliper guide grease. In addition, all mounting and noise-dampening hardware should be replaced. When installed, the caliper should be securely mounted. If a bushing or shim has been left out, the caliper will rattle on rough roads or make a “chucking” noise when traveling over tar strips on paved roads.

If the brake hose has been removed from the caliper, it’s important to install new copper washers on the brake hose connection. In some cases, a work-hardened or slightly damaged washer can allow air to leak into the brake hose and cause a low-brake pedal complaint to develop. Next, it’s important to make sure that the brake hose is the correct length and does not contact the frame, steering or suspension components at the extremes of steering and suspension travel.

SAFETY FIRST
When performing any brake service, it’s doubly important to make sure that all brake system and related components meet OE specifications and will perform in a safe, reliable manner. Allowing a vehicle to leave the shop with new brake pads mounted on excessively machined brake rotors or on defective brake calipers can become a potential liability if the vehicle is involved in an accident and comes under the scrutiny of an accident investigation team. Think “safety first” and you’ll never be sorry.

Gary Goms owns Midland Engine Electronics & Diagnostics in Buena Vista, CO. He is an ASE-certified Master Auto Technician, ASE L-1 Advanced Engine Performance Technician, and specializes in performance-related diagnostic issues.

 
 
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