Brake Pull Diagnosis

Brake Pull Diagnosis

Problem: Vehicle pulls to one side during braking

Cause: There are many causes of brake pull. here is a list of possible causes:

  • 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

Solution: Determining the actual cause of the pull condition relies on approaching the problem in a methodical fashion. When faced with a brake pull condition follow these steps:

1. Confirm pull condition (see Figure 56.1). Drive vehicle on typically crowned roads. Note vehicle handling during both braking and non-braking. If pull is duplicated proceed to next step.

how to bleed brakes Figure 56.1

2.  Before suspecting brakes check tire size, condition and air pressure. If not within acceptable levels side to side swap tires accordingly and road test vehicle. If pull is still present go to next step.

3.  Perform an inspection of the steering and suspension components. Pay close attention to those that will allow a toe or caster change during braking. If any component shows excessive wear that could contribute to the pull repair is suggested before continuing. If front end parts are tight go to next step.

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

5.  Perform brake inspection. Pay close attention to the operation of the calipers. Check caliper housing’s ability to move freely. Check piston condition by pulling back dust boot. If significant difference exists from side to side service calipers and road test vehicle. If pull is still present go to next step.

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

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

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

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

10.  The most common point of restriction are 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. If corrosion is present pry the bracket apart, squeeze the hose in the direction opposite the crush pattern and then test drive the vehicle. If the pull is eliminated or substantially reduced the brake hose is the cause. If no external signs of restriction are visible the decision has to be made to replace the hose(s) or perform additional diagnostic steps. Those steps could include switching hoses from side to side or trying to blow 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 be the cause of the pull by diagnosis or replacement the system should be checked for an upstream restriction. Go to next step.

11.  Brake pull still present after performing steps 1 to 10 – This should not happen very often. If steps 1 to 10 have been followed correctly and the vehicle is still exhibiting a brake pull then identify hydraulic components between 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 could be switched using the necessary line adapters to determine if valve is source of restriction. If pull is still present after step 11 it is advisable to recheck previous steps.

Screen Shot 2016-05-10 at 10.21.06 AM

Diagnosing Two Wheel Drag – Disc Brakes

Diagnosing Two Wheel Drag – Disc Brakes

Problem

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:
 
Diagram
  • 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

Problem

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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Measuring Wheel Drag

Measuring Wheel Drag

Wheel Drag: The Problem

Accurately determining wheel drag during diagnosis

The Cause of Misdiagnosed Wheel Drag:

Many mechanics judge the amount of drag by rotating the tire/wheel assembly by hand, which is not always an accurate method of determining drag.

How to Measure Drag:

When diagnosing the cause of a drag check, it is essential to accurately determine if the results of a test step impacted the amount of drag. Some drag conditions result in only a slight drag, making diagnosis more difficult.

The most accurate method to determine the drag on a wheel is to use an inch-pound torque wrench. This method is also the best method to use when trying to determine the amount of change on drag during a diagnostic test. To perform a drag test using an inch-pound torque wrench, follow the steps below:

1. Duplicate the drag condition
2. Adjust the torque wrench to a midway setting such as 30 (Figure 53.1)

Torque Wrench set to mid setting

53.1

3. Position a wheel stud to the three o’clock position as shown in Figure 53.2

Wheel stud at 3'oclock

4. Place the torque wrench on the lug nut at the three o’clock position.

5. Attempt to rotate the tire/wheel assembly while noting whether or not the torque wrench clicks. If the torque wrench clicks before the wheel turn, go to the next step. If the torque wrench does not click before the wheel rotates, go to step 7.

6. Torque wrench clicks before wheel rotates – adjust the torque wrench to a higher setting and attempt to turn the wheel. A good rule of thumb is to increase it by 10. Repeat this step until the wheel rotates when the torque wrench clicks.

7. Wheel rotates before torque wrench clicks – adjust the torque wrench down and rotate the wheel. A good rule of thumb is to decrease it by 10. Repeat this step until the wheel turns when the torque wrench clicks.

TIP: When diagnosing a one-wheel drag condition, measure the drag on the opposite (non-dragging) wheel to use as a comparison. When performing diagnostic steps to determine the cause of a drag, use the opposite wheel’s drag measurement to determine if the dragging wheel is completely releasing. See page 94 for one-wheel drag diagnosis

Tip: while most wheel drag problems are easy to identify some are not. Knowing how to determine the amount of drag is necessary when diagnosing certain types of problems.

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Red Brake Warning Light On

Red Brake Warning Light On

Problem:

This article will go over what to do when the red brake warning light comes on after service on vehicles equipped with pressure differential switch.

Red Brake Warning Light - Cause:

The pressure differential switch piston is offset, causing the pressure differential switch to remain in a grounded position.

How to Fix the Red Brake Warning Light:

The solution applied will depend on what type of pressure differential switch the vehicle has. Most pressure differential switches have only one function – turn the red warning light on if a hydraulic failure occurs in either the primary or secondary circuits. Some others have the additional function of limiting fluid loss to the rear brake circuit in a rear circuit failure.

Conventional pressure differential switch (fluid flow to rear brake):

Bleed the system using the manufacturer’s sequence until all the air is out of the system. If the pressure differential switch does not re-center itself during bleeding, it creates a pressure loss opposite the original hydraulic failure. For example, if the initial failure were a broken rear brake line, you would open either front bleeder to create this difference.

Once open, spike the brake pedal a couple of times while watching the red warning light (key on), using caution not to exceed ¾ pedal travel to prevent master cylinder damage. The pressure difference should push the piston back to the center.

Special purpose pressure differential switch (restricted fluid flow to rear brakes):

This type of pressure differential switch will not allow a system to bleed until the piston is re-centered. To re-center, bleed the system from the master cylinder down to the valve’s inlet until you can no longer see air. Accomplish this by cracking the line fitting at both the master outlet and valve inlet.

Once bled, spike the brake pedal a couple of times while watching the red warning light (key on), using caution not to exceed ¾ pedal travel to prevent master cylinder damage. The pressure difference should push the piston back to the center. Once re-centered, complete the repair by bleeding the rear brakes.

More Info on Red Brake Warning Light:

To better understand the fix, an understanding of the pressure differential switch’s function is in order. Figure 51.1 is a cross-sectional view of a stand-alone pressure differential switch. The pressure differential switch consists of a piston exposed to primary and secondary circuit pressures and a normally open switch. The switch consists of the switch body, a spring-loaded plunger, and a contact pin.

Tip: Knowing what can turn the red brake light on is the first step in determining how to get it to go off.
Schematic

51.1

The piston has what is called a detent in the center of it. The detent is a beveled indentation where the switch plunger rests. The switch plunger is held in place by the spring tension of the switch.

Each end of the piston is fitted with o-rings to form a seal from the primary and secondary circuit pressures. One end of the piston is exposed to primary circuit pressures, while the opposite end has exposure to secondary circuit pressures.

Regular system operation produces near equal pressures in both the primary and secondary hydraulic circuits. The spring-loaded plunger on the switch keeps the piston from moving under minor pressure differences. Typically it will prevent movement until the pressure difference reaches approximately 150 psi.

If a hydraulic failure occurs in either the primary or secondary circuits, it will create a pressure difference. The high pressure on one side of the piston will push the piston towards the failed side or low-pressure circuit (See Figure 51.2).

The movement of the piston will cause the open switch to go to a closed position which completes the ground to the red warning light. The light will remain on until the piston is re-centered.

Figure 51.2

Hydraulic Failure Causing Red Brake Warning Light

Most vehicles will never experience a hydraulic failure, and therefore the pressure differential piston will never move. This lack of movement is responsible for the piston “sticking” when offset due to a hydraulic failure. The piston bore corrodes and causes an interference fit.
Special Purpose Pressure Differential Switches

A group of specialized pressure differential switches has an additional function in a rear circuit failure. In addition to turning the red warning light on, they also restrict fluid flow to the rear brakes in the rear circuit hydraulic failure vent.

A typical method is in Figure 51.3. Figure 51.3 shows the piston in the neutral position. Fluid flow to the rear brakes is through the passages marked as such. Figure 51.4 shows what happens to these passages when a rear circuit hydraulic failure occurs. The piston offsets and covers the holes. As a result, fluid flow will be restricted to the rear circuit, limiting fluid loss.

Fluid passages to rear brakes

51.3

51.4

If a rear circuit hydraulic failure occurs and the piston does not re-center, the rear brakes will not be bled until the piston is re-centered. See procedure under “Solution.”

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Metering Valve Diagnosis

Metering Valve Diagnosis

Problem:

If you’re experiencing premature pad wear, nose-diving, or lack of stopping power on vehicles equipped with a metering valve, this is the right article for you.

Metering Valve Cause:

As you consider the cause of any of the problems above, it is essential to add the metering valve to the list of possible reasons for the above issues. The symptoms produced by failed metering valves will depend on the type of failure and type of vehicle. Here is a description of each type of failure:

1 – Valve stuck OPEN: If the valve is stuck open, the vehicle will have instant-on front brakes (See figure 48.1). The front brakes will apply instantly, while the rear brakes will have to “play catch up.” The long-term symptom of instant-on front brakes is premature front pad wear on all pads. Depending on the vehicle, this condition may also cause the vehicle to nose dive when stopping. This symptom does not always present itself with valve failure.

Corroded stem cap

48.1

2 – Valve stuck where it can’t open completely: if the restriction in the valve’s movement is such that it can only open partway or seized in this position, the vehicle will only have partial front braking. The typical complaint from the driver will be a lack of stopping power.

Metering Valve Solution:

The metering valve should be one of the first things checked when diagnosing the above complaints. The reason for this is it will be one of the easiest things to check. Perform the following steps to verify metering valve function:

1 – Locate the metering valve. It will either be under the vehicle or the hood. Check the external condition of the cap screw–stem area as shown in Figures 48.2 & 48.3.

Figure 48.2 shows an underhood valve. Metering valves located under the hood rarely fail because of the protection they have from the elements. This valve does not need checking. The valve in Figure 48.3 is mounted under the vehicle and receives exposure to the elements. The cap screw and stem have been bonded together by the corrosion.

Metering Valve

48.2

48.3

2 – If the metering valve is in question, check it by having someone apply the brake pedal while watching the metering valve stem. The stem should move out on use and in on release, as shown in Figures 48.4 & 48.5.

Metering Valve

48.4

Metering Valve Stem Out

48.5

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Brake Job Done Well

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