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The Gentleman's Express: Tech-Tips from the JIOC

Table of Contents

Brake System Discussion

Author: reprinted from: IAPA/October 1985 by John Ayers

Brake System Components

The more knowledge people have, the more confident they become.

Product knowledge not only concerns what parts make up a brake system, but it also addresses a clear understanding of how those parts work together to successfully and safely bring a vehicle to a stop.

System Overview:

As the pedal is depressed, the master cylinder is operated by the pedal linkage or the power brake booster, which has been activated by the pedal. The master cylinder then delivers pressurized fluid to the wheel brakes through steel lines, brake valving, and rubber hoses. The pressurized fluid actuates disc brake calipers which in turn force friction materials against rotors. The rotors, turning with the wheels, are slowed by contact with the friction material, hereby slowing or braking the vehicle.

The brake pedal is simply a lever, multiplying the driver’s effort. No brake diagnosis is complete without checking for pedal “free”. An excessively low pedal, a spongy pedal, or a pedal that slowly “creeps” to the floor when depressed indicates defects in the brake system. Most power brake boosters utilize engine manifold vacuum to multiply the driver’s braking effort. These boosters are not easily serviced and should be replaced if defective. Remember that a faulty vacuum source is often the culprit when a booster fails to perform. The master cylinder is an hydraulic pump operated by the brake pedal and the power brake unit. Modern master cylinders are of dual-system design, containing two separate hydraulic pumps: a primary system and a secondary system. The two systems provide a measure of safety, for if one system fails, the other will remain functional.

The master cylinder should always be inspected for leaks or external damage. Never fail to remove the reservoir caps to check the condition and level of the brake fluid. There are too many variations of master cylinder design to discuss in one article. However, when you encounter a master cylinder or other brake component unfamiliar to you, consult a qualified technician or the manufacturer’s service manual for guidance. Remember that most master cylinders require bench bleeding prior to installation. This is an important time-saving step in master cylinder service.

Brake Fluid:

Brake fluid, itself, is often overlooked. It cannot be emphasized enough as to the importance of using clean, recommended fluid from sealed containers. There is no advantage in replacing old, contaminated fluid with new fluid if it, too, is contaminated. Only use sealed fluids recommended by the manufacturers of the brake systems and brake replacement products for maximum life expectancy of brake work. Brake fluid should be replaced when servicing the brake system to retard the corrosive effects of moisture accumulated in the old fluid.

Two common types of brake valving are the metering valve and the proportioning valve. The metering valve is primarily used in conjunction with front disc brakes on full size disc/drum systems to compensate for weight transfer to the front axle during braking. The proportioning valve is commonly used to reduce rear wheel braking pressure. This prevents lockup during hard braking applications. These valves are not serviceable should they become faulty. A new valve should be installed, but both types of valve seldom need replacing.

Metal lines carry brake fluid to the wheels. These lines are subject to high pressure fluid and other stresses, therefore, it is always best to use steel lines that are designed for use in the brake system. If it is necessary to cut a steel line during installation, the proper flaring tools should be used to make good fits on the connections.

Rubber hoses are used to join the metal lines to the calipers and the steel cylinders. These are flexible to allow for the motion of the suspension and the turning of the wheels. Any visible defects would warrant replacement of the hose. Hoses with internal damage can only be determined as such by a thorough diagnosis. Front brake hoses should be replaced as a set.

Calipers should be rebuilt when disc pads are replaced to insure free movement of the piston and maximum pad life. If the caliper is not serviced as such, the result may be brake drag which causes premature wear of the brake pads and possibly brake failure.

Disc Brakes:

Disc brake hardware also needs routine replacements. The hardware holds the caliper squarely over the rotor, controls the sliding action of the caliper, and reduces vibration. Weakened hardware can cause uneven disc pad wear and brake noise.

Disc brake rotors should be machined to produce an even finish and to true the rotor, aiding in the prevention of pedal pulsations, brake noise, and promoting pad break-in. To protect the machined rotor, use only a torque wrench to tighten the lug nits when re-installing the wheel. Improperly torqued lug nuts are a main cause of rotor warpage on new brake jobs. It is just as important to promote the torqueing of lug nuts on new brake jobs as any other information given in connection with proper brake service.

There are primarily two types of disc brake pads in common use, organic and semi-metallic. Semimetallic disc brake pads have become prevalent as original equipment on most cars and light duty trucks. This is due to the ability of semi-metallic brake material to withstand high temperatures associated with today’s brake systems. To insure safety, it is highly recommended that vehicles originally equipped with semi-metallic brake pads be serviced with semi-metallic disc pads that meet original equipment standards.

Rotor finish should be of the non-directional type. A normal brake lathe produces a directional finish not suitable for semi-metallic pad installation. This special technique refers to the pad break-in period. To retain their ability to absorb and withstand high temperatures, semi-metallic disc brake pads must be broken in during the first 150 to 200 miles of driving. This means that the brakes must not be subjected to needless abuse. This break-in period prevents glazing of the new pads which causes disc brake squeal.

Parking Brakes:

Last but not least, the matter of the components that make up the parking brake should be addressed. Many mechanics have overlooked that aspect of brake service and, as a result, have had to deal with countless comebacks because of brake drag due to corroded, or malfunctioning parking brake components. Therefore, the parking brake must be serviced as would any other component in the brake system.

John Ayers is a Director, Sales & Marketing Div. Bendix Aftermarket Brake Division.

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