Brakes and Traction Control: Testing and Inspection

BASE BRAKE SYSTEM

The base brake system components consist of front and rear brake pads and calipers. Rear park brake drums and shoes which are internal to the rear rotor. Front and rear brake rotors and brake lines. The master cylinder, booster and the Hydraulic Control Unit (HCU).

Brake diagnosis involves determining if the concern is related to a mechanical, hydraulic, electrical or vacuum operated component.

The first diagnosis step is the preliminary check.

PRELIMINARY BRAKE CHECK

(1) Check the condition of the tires and wheels. Damaged wheels and worn, damaged or under inflated tires can cause a pull, shudder, vibration and a condition similar to grab.

(2) If a complaint was based on noise while braking, check the suspension components. Jounce the front and the rear of vehicle and listen for anything that might be caused by a loose, worn or damaged suspension or steering component.

(3) Inspect the brake fluid level and condition. Note that the brake reservoir fluid level will decrease in proportion to normal lining wear. ALSO NOTE THAT BRAKE FLUID TENDS TO DARKEN OVER TIME. THIS IS NORMAL AND SHOULD NOT BE MISTAKEN FOR CONTAMINATION.

(a) If the fluid level is abnormally low, look for any evidence of leaks at the calipers, brake lines, master cylinder and at the HCU.

(b) If the fluid appears to be contaminated, drain out a sample to examine. The system will have to be flushed if the fluid is separated into layers, or contains a substance other than brake fluid. The system seals, cups, hoses, master cylinder and HCU will also have to be replaced after flushing. Use clean brake fluid to flush the system.

(4) Check the parking brake operation. Verify free movement and full release of the cables and the lever. Also note if the vehicle was being operated with parking brake partially applied.

(5) Check the brake pedal operation. Verify that pedal does not bind and has adequate free play. If the pedal lacks free play, check the pedal and the power booster for being loose or for a bind condition. DO NOT road test the vehicle until the condition is located and corrected.

(6) Check the boosters vacuum check valve and hose.

(7) If the components checked appear to be OK, road test the vehicle.

ROAD TESTING

(1) If the complaint involved a low brake pedal, pump the pedal and note if it comes back up to the normal height.

(2) Check the brake pedal response with the transmission in neutral and the engine running. The pedal should remain firm under constant foot pressure.

(3) During the road test, make normal and firm brake stops in the 25-40 m.p.h. range. Note any faulty brake operation such as: low pedal, hard pedal, fade, pedal pulsation, pull, grab, drag, noise, etc.

(4) Attempt to stop the vehicle with the parking brake only. DO NOT exceed 25 m.p.h. Note any grab, drag, noise, etc.

PEDAL FALLS AWAY

A brake pedal that falls away under steady foot pressure is generally the result of a system leak. The leak point could be a brake line, fitting hose or caliper. If the leaking is severe, fluid will be evident at or around the leaking component.

Internal leakage (seal by-pass) in the master cylinder is caused by worn or damaged piston cups, this could also be the cause of the problem.

If no visual leaks are apparent, an internal leaking ABS system may be the cause of the problem.

LOW PEDAL

If a low pedal is experienced, pump the pedal several times. If the pedal comes back up, the most likely cause are calipers that are not sliding on the slide pins. The proper course of action is to inspect and replace all the worn components.

SPONGY PEDAL

A spongy pedal is most often caused by air in the system. However, substandard brake hoses can cause a spongy pedal. The proper course of action is to bleed the system, and replace the substandard quality brake hoses if suspected.

HARD PEDAL OR HIGH PEDAL EFFORT

A hard pedal or high pedal effort may be due to a brake pad that is water soaked, contaminated, glazed or badly worn. The power booster, check valve, check valve seal/grommet or a vacuum leak could also cause hard pedal or high pedal effort.

PEDAL PULSATION

Pedal pulsation is caused by components that are loose or beyond tolerance limits.

The primary cause of a pulsation are, disc brake rotors with excessive lateral runout or thickness variation, loose wheel bearings or calipers and worn or damaged tires.

NOTE: Some pedal pulsation may be felt during ABS activation.

BRAKE DRAG

Brake drag occurs when the brake pads are in constant contact with the rotor. Drag can occur at one wheel, all wheels, fronts only or rears only.

Drag is a product of an incomplete brake release. Drag can be minor or severe enough to overheat the brake pads, rotors, and park brake drums.

A minor drag will usually cause slight surface charring of the brake pads. It can also generate hard spots in the rotors from the overheating-cooling down process. In most cases, the rotors, wheels and tires are quite warm to the touch after the vehicle is stopped.

A severe drag can char the brake pads all the way through. It can also distort and score the rotors to the point of replacement. The wheels, tires and brake components will be extremely hot. In severe cases, the brake pads may generate smoke as it chars from over-heating.

Common causes of brake drag are:

- Parking Brake partially applied
- Loose/warn wheel bearings
- Seized caliper
- Caliper binding
- Loose caliper mounting
- Mis-assembled components
- Damaged brake lines

If the brake drag occurs at the front, rear, or at all wheels, the problem may be related to a blocked master cylinder return port, faulty power booster (binds does not release) or the ABS system.

BRAKE FADE

Brake fade is usually a product of overheating caused by brake drag. However, brake overheating and resulting fade can also be caused by riding the brake pedal, making repeated high deceleration stops in a short time span, or constant braking on steep mountain roads. Refer to the Brake Drag information.

BRAKE PULL

A front brake pull condition could result from:

- Contaminated brake pad on one caliper
- Seized caliper piston
- Binding caliper
- Loose caliper
- Rusty caliper slide surfaces
- Improper brake pads
- Damaged rotor
- Wheel alignment
- Tire pressure

A worn, or damaged wheel bearing, or worn suspension components are other causes of a pull. A damaged front tire (bruised, ply separation) can also cause a pull.

A common and frequently misdiagnosed pull condition is where direction of pull changes after a few stops. The cause is a combination of brake drag followed by fade at one of the brake units.

As the dragging brake overheats, efficiency is so reduced that fade occurs. Since the opposite brake units are still functioning normally, its braking effect is magnified. This causes the pull to switch direction in favor of the normally functioning brake unit.

An additional point when diagnosing a change in pull condition concerns brake cool down. Remember that the pull will return to the original direction, if the dragging brake unit is allowed to cool down (and is not seriously damaged).

REAR BRAKE DRAG OR PULL

Rear drag or pull may be caused by an improperly adjusted park brake shoe or a seized parking brake shoe or a seized parking brake cable, contaminated lining, bent or binding shoes or improperly assembled components. This is particularly true when only one wheel is involved. However, when both rear wheels are effected, the master cylinder or ABS system could be at fault.

BRAKES DO NOT HOLD AFTER DRIVING THROUGH DEEP WATER PUDDLES

This condition is generally caused by water soaked brake pads. If the brake pads are only wet, they can be dried by driving with the brakes lightly applied for a mile or two. However, if the lining is both soaked and dirt contaminated, cleaning and or replacement of the pad will be necessary.

BRAKE LINING CONTAMINATION

Brake pad contamination is mostly a product of leaking calipers or worn seals, driving through deep water puddles, or a pad that has become covered with grease or grit during repair. A contaminated pad should be replaced to avoid further brake concerns.

WHEEL AND TIRE PROBLEMS

Some conditions attributed to brake components may actually be caused by a wheel or tire concern.

A damaged wheel can cause a shudder, vibration and a pull. A worn or damaged tire can also cause pull.

NOTE: Propshaft angle can also cause vibration/shudder.

Severely worn tires with very little tread left can produce a grab-like condition as the tire loses and recovers traction. Flat spotted tires can cause vibration and generate shudder during brake operation. Tire damage such as a severe bruise, cut, ply separation, low air pressure can cause pull and vibration.

BRAKE NOISES

Some brake noise is common on some disc brakes during the first few stops after a vehicle has been parked overnight or stored. This is primarily due to the formation of trace corrosion (light rust) on metal surfaces. This light corrosion is typically cleared from the metal surfaces after a few brake applications causing the noise to subside.

BRAKE SQUEAK/SQUEAL

Brake squeak or squeal may be due to pads that are wet or contaminated with brake fluid, grease or oil. Glazed pads and rotors can also contribute to squeak, Dirt or foreign material embedded in the brake pads will also cause brake squeal.

A very loud squeak or squeal is frequently a sign of severely worn brake pads. If the lining has worn through the brake pad in spots, metal-to-metal contact occurs. If the condition is allowed to continue, rotors may become so scored that replacement is necessary.

BRAKE CHATTER

Brake chatter is usually caused by loose or worn components, or glazed/burnt linings. Rotors with hard spots can also contribute to chatter. Additional causes of chatter are out-of-tolerance rotors, brake pads not securely attached to the calipers, loose wheel bearings and contaminated brake pads.

THUMP/CLUNK NOISE

Thumping or clunk noises during braking are frequently NOT caused by brake components. In many cases, such noises are caused by loose or damaged steering, suspension or engine components.