Wheel Bearing Diagnostic Tips

Wheel Bearing Diagnostic Tips


Coming up with an accurate diagnosis for unitized style wheel bearing.


The sealed design of the wheel bearing sometimes makes identifying which bearing is the source of the complaint difficult.


There are several techniques available that can help pinpoint a failing bearing. They are listed below:

Test Drive

Wheel bearings can cause many different types of noises when driving. The noise will usually be more pronounced when the bearing is in a loaded condition, such as during a turn.

Vibration Test

Failing wheel bearings will often cause a roughness during wheel rotation. The vehicle’s strut and spring act like an amplifier and allow easy diagnosis of a rough bearing. Place your hand on the spring and rotate the wheel assembly. A rough bearing will produce a pronounced vibration through the spring. Repeat the procedure on the opposite side.

Stethoscope Test

Failing bearings will often make a noise when they start to fail. The noise is not always easy to identify. The use of a stethoscope can make pinpointing the problem bearing easier. Once the vehicle is both on the rack and in gear, you can use the stethoscope. Start by placing the stethoscope on the knuckle as close to the bearing as possible. If the bearing is failing, it will produce a noticeable hum or grinding sound. It may be necessary to turn the wheel slightly to duplicate the bearing noise on the rack.

Caution: It is essential to take extreme care when working near a rotating wheel.

Isolation Test

In some cases, it will be necessary to isolate the bearing to determine its condition. For instance, for most FWD vehicles, this is a relatively straightforward process. First, remove the caliper and rotor. Next, disconnect the outer tie rod end and low ball joint. Continue by removing the CV axle nut. Next, disconnect the knuckle from the lower ball joint and remove the CV joint from the hub. The bearing is now isolated. Finally, rotate the bearing while feeling for roughness.

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Identifying When Self-Adjustment Takes Place on Drum Brake Vehicles

Identifying When Self-Adjustment Takes Place on Drum Brake Vehicles

The purpose of this article is to help mechanics like you avoid improper rear drum brake adjustments. As you read, take note of the different common mistakes and how to stay away from them. Likewise, you may notice that there are several ways to ensure a quality job.

Rear Drum Brake Adjustments

One common problem when dealing with rear drum brakes is that mechanics don’t understand when and how self-adjustment occurs. As a result, improper adjustments can occur. On the other hand, learning to identify the self-adjusting parts and when they operate will allow the following:

  • Determining whether the self-adjusting mechanism is operating as designed.
  • Allow proper assembly or repair of self-adjustment mechanism
  • Identify when self-adjustment takes place

Duo-Servo Drum Brake

You can identify duo-servo drum brakes by the location of either the self-adjuster or anchor pin. Likewise, the location of the self-adjuster is at the bottom between the primary and secondary brake shoes. Likewise, the anchor pin is at the top between the two brake shoes.

All duo-servo drum brakes are designed to self-adjust when backing up only under the following conditions:

  • There is a large enough gap between the secondary brake shoe and the brake drum.
  • The parts that make up the self-adjusting mechanism are operating correctly.

When servicing vehicles with duo-servo drum brakes, it is essential to make sure all parts are lubricated and installed correctly.


Non-Servo Drum Brake

Often times FWD vehicles have non-servo drum brakes. As a result, the most distinguishing feature of a non-servo drum brake is the location of the anchor pin. For example, the anchor pin on non-servo drum brakes is at the bottom of the backing plate.

There are two techniques used to allow self-adjustment on non-servo drum brakes. The most common is during any forward braking. For the vast majority of FWD vehicles use this method. There are a small number of non-servo drum brakes that use the parking brake to self-adjust.

A good rule of thumb is to use the self-adjuster’s location to help determine when self-adjustment takes place. In the case that you notice that the self-adjuster is part of the strut rod self-adjustment occurs during forward braking. For instance, If the self-adjuster is part of the parking brake lever then self-adjustment occurs during parking brake use.

While these rules of thumb are pretty accurate, they do not apply to all cases. Another way to determine when self-adjustment takes place is to engage both methods and see which one works.

Figure 60.1

Figure 60.1

Figure 60.2

Figure 60.2

Figure 60.3

Figure 60.3

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Brake Hose Tips

Brake Hose Tips

Brake Hose Problems

Internal brake hose damage during inspection or service

Brake Hose Problem 1:

Use of non-approved hose clamping devices such as vise grips (See Figure 58.1) Vise grips or similar tools can permanently damage the internal structure of even a new  hose.

Vice grips on a brake line

Brake Hose Problem 2:

Hanging caliper by the hose during inspection or service as shown in Figure 58.2. Hanging a caliper by the hose puts undue stress on the internal structure of the hose and can cause permanent damage. Some light truck calipers weigh as much as 18lbs. Calipers should be supported during service, as shown in Figure 58.3.

Cause Solution 1:

Use an alternate method for preventing fluid loss during service such as:

  • Use a tire valve stem to plug the brake hose at the banjo bolt opening.
  • Change the caliper only when the replacement part is available to prevent excessive fluid loss.

Cause 2 Solution:

Support caliper during inspection and service using pipe hook or another suitable tool. Do not rest the caliper on suspension or frame as it will probably fall off and could create even more damage to the hose.

More Info: The brake hose is made up of a series of layers of rubber and a woven fabric. These layers give the hose the ability to withstand high pressures and prevent leaking. Failure to properly handle the hoses during inspection or service can result in ether a restricted hose or a one way check-valve condition.

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Brake Pull Diagnosis

Brake Pull Diagnosis


Brake pull occurs when a vehicle pulls to one side or the other during braking.


Depending on several variables, there are a variety of potential causes for brake pull. For example, most of the common causes have been listed below:

  • Friction problem
  • Rotor friction surface problem
  • Caliper problem
  • Restricted brake hose
  • Hydraulic restriction higher than brake hose
  • Rear brake imbalance
  • Loose front end part
  • Alignment
  • Tires



Determining the actual cause of the pull condition relies on methodically approaching the problem. When faced with a brake pull condition, follow these steps:


1. Confirm Brake Pull

First, drive the vehicle on typically crowned roads. Next, take note of how the vehicle handles both braking and non-braking. If you notice any brake pull, proceed to the next step.

Red truck with brake pull

2. Test more parts to rule out brake pull

Before jumping to conclusions, it is essential to check tire size, condition, and air pressure. If any tires are not within acceptable levels, side to side swap tires accordingly and road test the vehicle. Otherwise, if the pull is still present, go to the next step.

3. Inspect steering and suspension

Pay close attention to any part that will allow a toe or caster change during braking. If any component shows excessive wear that could contribute to the pull, the repair is suggested before continuing. Otherwise, if all front-end parts are tight, go to the next step.

4. Line lock test

Line lock rear brakes with approved line lock. Test drive vehicle to check if the pull is affected. If the pull is gone, the cause is in rear brakes. If the pull is still present go to the next step.

5. Brake Inspection for brake pull

Pay close attention to the operation of the calipers. Check caliper housing’s ability to move freely. Next, check piston condition by pulling back dust boot to see if a significant difference exists from side to side service calipers and road test vehicles. If the pull is still present, go to the next step.

6. Swap brake pads

In the case that the front pads have friction material left on them, swap brake pads from side to side and test drive the vehicle. If the pull is gone or switches, directions friction is the source of your problem. If there is no change in the pull, swap the brake pads back to their original position and go to the next step.

7. Swap Rotors

With brake pads back to their original position, swap rotors, and test drive vehicle. If the pull is gone or switches sides rotor’s friction surface is the cause. If there is no change in the pull go to the next step.


8. Brake pull Side-to-side pressure check

If pressure gauges or clamping plate gauges are available, perform aside to side pressure check. Pressure readings should be within 50psi at a pressure above 500psi. If pressure readings are not within 50psi of one another, go to the next step, or if pressure gauges are not available to proceed to the next step.


9. Determine brake pull restriction points

Determine possible points of restriction. On a front to rear split hydraulic system this will include the brake hose, ABS modulator (if equipped), and combination valve (rare but possible), and steel brake line to the side opposite the direction of the pull. If the diagonal split hydraulic system, the list will include brake hose, ABS modulator (if equipped), steel brake line to the side opposite the direction of the pull, and master cylinder.

10. Brake hoses and brake pull

The most common point of restriction is the brake hoses. The hose on the side opposite the direction of pull is the most likely cause. If the hose has a mounting bracket clamped around it, check the bracket for signs of corrosion. When corrosion is present, pry the bracket apart, squeeze the hose in the direction opposite the crushed pattern and then test drive the vehicle.

In the case that the pull is eliminated or substantially reduced, the brake hose is the cause. If no external signs of restriction are visible, the decision must be made to replace the hose(s) or perform additional diagnostic steps. Those steps could include switching hoses from side to side or blowing through both hoses to check if there is a difference.

Usually, the time necessary to do either test would be better spent trying a new set of hoses. If the hoses are shown not to cause the pull by diagnosis or replacement, the system should be checked for an upstream restriction. Go to the next step.

11. Hydraulic components

Brake pull still present after performing steps 1 to 10 – This should not happen very often. If you followed steps 1 to 10 correctly and the vehicle is still exhibiting a brake pull, identify hydraulic components between the master cylinder and front brakes.

These will usually include the ABS modulator and/or the combination valve. On vehicles equipped with combination valves, the outlet lines of the valve can switch using the necessary line adapters to determine if the valve is the source of restriction. If the pull is still present after step 11, it is advisable to recheck previous actions.


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Diagnosing Two Wheel Drag – Disc Brakes

Diagnosing Two Wheel Drag – Disc Brakes


Both front wheels are experiencing wheel drag.

Cause of Two Wheel Drag

Residual pressure causing brakes to apply after brakes reach operating temperature. As a result, the most common cause for this condition comes from plugged or covered vent ports in the master cylinder.

Two Wheel Drag Solution

The first step to fixing two-wheel drag is to identify the cause of the residual pressure. As you’ll see, when presented with a two-wheel drag complaint, the diagnostic process starts under the hood, not at the wheel. Similarly, both wheels having the same symptom rules out the calipers and brake hoses as likely causes. The cause will almost always be something similar to both wheels (See Figure 55.1).

NOTE: Before proceeding with diagnosis check the system for contamination.

You will need to duplicate the system before the diagnosis can continue. To begin, take the vehicle for a test drive to bring the brakes up to operating temperature. When performing this step, it is best to follow the advice below:
  • First, be sure to stay close to the shop.
  • Next, take a line wrench (If the drag increases to a non-drivable point, you’ll be glad you have it.)
  • Finally, do not “hammer” the brakes during the test drive. Make numerous stops in city driving until you either feel the drag or know the brakes are at operating temperature.

Diagnosing Two Wheel Drag

1. Once the brakes are at operating temperature or the drag has been duplicated, rack the vehicle and leave it neutral.


2. Check all four wheels for drag. If necessary, measure drag.


3.  Drag on front brakes (or front and rear on four-wheel disc brake-equipped vehicles) – first, start by loosening the master cylinder away from the vacuum booster by ¼” . Check effect on drag condition. If drag is still present, go to the next step. If drag is released, go to step eight.


4. Drag still present with master away from booster – Loosen the brake line(s) supplying the dragging brakes. Check effect on drag. If drag is released, there is an internal problem in the master cylinder. You’ll either have plugged vent port(s) or a binding piston(s) in the bore. Master cylinder replacement is required. If drag is not released, go to the next step.


5. Drag is not released after brake lines at master cylinder are loosened. First, locate the next component downstream from the master cylinder. Likewise, trace the brake lines from the front wheels to that component. Next, loosen outlet lines of components that supply the dragging brakes. If drag is released component is the source of restriction and requires replacement. If drag is not released, go to the next step.

Drag Not Releasing

6.  Drag is not released after loosening brake lines at the outlet of the component between the master cylinder and dragging brakes. Trace lines to dragging brakes. For instance, is there another component between the component tested in step 5 and dragging brakes? If yes, repeat step 5 on that component. If no, go to the next step.

7.  Wheel drag still present on both wheels after performing steps 3 through 6. Open bleeder screws on both front calipers and check the effect on drag condition. If either wheel frees up, the brake hose on that wheel acts as a one-way check valve. If either or both wheels fail to release, the problem is a mechanical problem with both front calipers. Inspect and service as required.

8. Wheels released after the master cylinder was moved away from the vacuum booster. The problem is in front of the master cylinder. Something is not allowing pistons in the master cylinder to return to a fully released position resulting in the cup seals covering the vent ports. The list of possible causes varies from vehicle to vehicle-based on system configuration. Here is a list of possible causes (see Figure 55.2):

Diagram 2

Possible Causes of Two Wheel Drag

  • Stoplight switch adjustment.
  • Binding pedal linkage  – check for unrestricted movement. If necessary, disconnect the booster pushrod.
  • Pedal height adjustment – some import vehicles have an adjustable link between the brake pedal and booster.
  • Vacuum booster applying partial assist – air and vacuum valves inside booster may not be returning to their proper position when the brake pedal is released. Likewise, pull the one-way check valve out of booster with drag present to check for this condition.
  • The adjustable pushrod on the booster is too long.

NOTE: Most 2 wheel drag problems will come from a problem in front of the master cylinder as described above or by the master cylinder itself. Likewise, problems below the master cylinder rarely cause Two-wheel drag problems.

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One Wheel Drag Diagnosis – Disc Brake

One Wheel Drag Diagnosis – Disc Brake


One wheel drag on disc brakes.

Cause of One Wheel Drag

There are several different possible causes for one wheel drag. The cause of a one-wheel drag on a disc brake can be either mechanical or hydraulic. The list of possible causes will include:

  • Caliper slides
  • Caliper piston
  • Check valve brake hose
  • Restriction in ABS modulator (if equipped)
  • Plugged or covered vent port (FWD only)

One Wheel Drag Solution

Applying a systematic approach to the diagnosis of a one-wheel drag condition will determine the cause. The diagnosis’s objective is to determine if the source is mechanical or hydraulic but to determine the actual cause.

To diagnose a one wheel drag on a disc brake, follow the steps below:


1. Duplicate the problem. Confirm the problem before continuing. If necessary, measure wheel drag.


2. Loosen the brake line fitting at the INLET end of the brake hose supplying the dragging wheel. Check the drag condition if the drag releases; go to step 5. If the drag is still present, go to the next step.

NOTE: If it is impossible to loosen the brake hose inlet fitting go to the next fitting upstream. You can usually find a fitting higher than the hose you can loosen without damaging the brake line.

3. Tighten brake line fitting. Loosen bleeder screw on dragging caliper. If the drag releases, the brake hose is the source of the problem and requires replacement. It is acting as a one-way check valve. Fluid is allowed to the caliper on applying but not allowing full release (see Figure 54.1). If the drag does not release, go to the next step.

Brake diagram

Figure 54.1

One Wheel Drag - Steps 4-7

4. If drag is still present after steps 1 to 3, the problem is mechanical in nature. The caliper piston or caliper slides are causing the drag. An inspection of the caliper should yield the source.

5. If drag is releasing after step 2, tighten the fitting and duplicate the problem. Locate the next component higher than the brake hose. This will most likely be the ABS modulator. With the wheel drag present, loosen the inlet brake line supplying the dragging wheel. Check effect on drag. If drag does not release, go to the next step. If drag does release, go to step 7.

6. Drag does not release after step 5 – loosen outlet brake line fitting at ABS modulator (or another component). Check drag. If released, the restriction is in the ABS modulator, and modulator replacement is required.

7. If drag is releasing after step 5 – tighten brake line fitting at the ABS modulator. Duplicate drag condition. Loosen inlet brake line fitting on next component upstream. This will usually be the combination valve on RWD vehicles and the master cylinder on FWD vehicles (See Figure 54.2) if the combination valve goes to step 8 if the master cylinder on the FWD vehicle goes to step 9.

One Wheel Drag - Steps 8-10

8. Combination valve on RWD vehicles – If loosening the inlet on the combination valve released, the drag goes to step 9. If wheel drag is still present, loosen the outlet line on the combination valve that supplies the dragging wheel. If the wheel releases, a combination valve is the source of restriction and will need replacing.

9. Tighten all fittings and identical drag conditions. With drag condition present, loosen master cylinder from power assist unit by at least ¼”. Check drag. If drag releases problem is in front of the master cylinder. This could include stoplight switch adjustment, pushrod adjustment, partial assist condition, or pedal height adjustment. If the drag condition is still present, go to step 10.

10. With drag condition present, loosen brake line fitting at master cylinder outlet supplying dragging brake. If the wheel frees up master cylinder is the source of drag and will require replacement.

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