Measuring Wheel Drag

Measuring Wheel Drag

by | Jul 2, 2021 | 101 Tech Tips, Brake System Tips

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

by | Jul 2, 2021 | 101 Tech Tips, Brake System Tips

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

by | Jul 2, 2021 | 101 Tech Tips, Brake System Tips

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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Proportioning Valve Diagnosis

Proportioning Valve Diagnosis

by | Jun 30, 2021 | 101 Tech Tips, Brake System Tips

Problem

Determining when and why to add the proportioning valves to the possible causes of the problem being diagnosed.

Cause

Not understanding the proportioning valve can result in the valve being added to the list of possible cases when it cannot cause the diagnosed problem or add it to the list when it should be.

Solution

 The proportioning valve is there to prevent rear-wheel lockup during panic braking. Its two main reasons are 1. most vehicles use drum brakes in the rear, and 2. all vehicles experience weight transfer during a panic stop. Rear drum brakes are hydraulic over mechanical in design.

If applying too much pressure to the wheel cylinders, lockup will occur. All vehicles with rear drum and rear disc brakes will experience weight transfer during a panic stop. When taking off the rear wheels’ weight during a panic stop, the tendency for the rear brakes to lock up will increase. The design of the proportioning valve is there to limit the pressure to the rear brakes during a panic braking situation.

When do proportioning valves work?

The most crucial point to understand about proportioning valves is when they work – only during panic braking. This means that a vehicle could potentially go its whole life without ever using its proportioning valve. Located between the master cylinder and rear brakes, the conventional proportioning valves are hydraulically activated.

Vehicles with front-to-back split hydraulic systems have one, while diagonal splits systems require two because each rear wheel is on a separate hydraulic circuit. The proportioning valve may be a stand-alone valve, part of the combination valve, or built into the master cylinder.

Deeper Inspection

Whether or not you take the additional steps to inspect or diagnose a vehicle’s hydraulic proportioning valve will depend on the following:
 
  • Could the customer’s complaint involve the proportioning valve?
  • What is the condition and location of the combination valve?
  • What type of proportioning valve does the vehicle have?
The only time that the proportioning valve should be on our list of possible causes if the customer’s complaint involves rear wheel lockup under panic braking. We must ask the customer when the rear wheel lockup is occurring – light to regular or heavy to panic braking?
 
If the answer is panic braking, the valve is on our list, but it won’t be the only thing on our list (See pg 104 for more information on rear wheel lockup). The valve’s design will determine how and when we are going to diagnose it.
 

Proportioning Valve Cap Screw

The design of some proportioning valves allows two things – more chance for failure and an easy way to diagnose them. If we look at the proportioning valve in Figure 49.1, we will see the cap screw that holds the piston and spring in place is vented.

The vent hole is a small rubber “mushroom” that prevents moisture from entering the cap screw. If this valve style is mounted low on the vehicle, as in most rear-wheel drive vehicles, it can introduce a high degree of moisture.

This is especially true in areas that use road salts for snow and ice removal. The moisture can corrode the cap screw and allow water to enter into where the piston travels in the cap screw (See Figure 49.2). This causes the cap screw to rust, which bonds the piston and cap screw together (See Figure 49.3).

Typically after removing the cap screw, the cap screw, spring, and piston are separate pieces. This corrosion prevents the piston from moving when a panic braking situation occurs.

Proportioning Valve

49.1

Cap Screw Vent

49.2

Piston Cap Screw

49.3

Checking the Proportioning Valves

The same thing that allows the valve to fail will enable us to check it – the cap screw vent hole. First, determine if the valve is a good candidate by its external appearance. If corrosion occurs outside the valve or the cap screw, it is a good candidate. Remove the rubber mushroom from the cap screw.

Next, insert a paper clip to the bottoms out of the piston, as in Figure 49.4. Have an assistant start the vehicle and apply heavy pedal pressure to simulate a panic braking situation. The paper clip should push against you and travel about 1/32” to 1/16” as the piston moves. If the paper clip doesn’t move, the piston has seized in the cap screw, and the valve will need replacing. This test will work on any proportioning valve vented through the cap screw and should be part of your standard inspection if the valve’s exterior condition merits it.

Checking the Proportioning Valves (Not Vented)

Bottom paper clip against piston

Many proportioning valves are not vented through the cap screw. There is not a quick check for these types of valves. These valves should only be checked if the customer’s complaint indicates a possible proportioning valve problem. Most shops do not have pressure gauges, so diagnosing the issue will have to be eliminated. You should check all other possible causes before condemning the proportioning valve.

Pressure gauges are the best way to diagnose a failed proportioning valve. There will be a gage installed into the front and rear hydraulic circuits. Start the vehicle, and apply the brake pedal with heavy pedal pressure to duplicate a panic braking situation. The front and rear pressures should be different. Most manufacturers do not publish these pressures. The rear brakes usually are “shut” down between 500psi and 800psi, while the fronts can climb to well over 1500psi See Figures 49.5, 49.6, and 49.7).

What you are looking for is whether the rear pressure has any limitation. If the rear pressure is over 1000psi and is close to or equal to the front pressure, then the valve is not working and will have to be replaced to correct the problem.

Proportioning valve operation - Normal braking

49.5

Knee Point

49.6

49.7

Proportioning Valves Rules of Thumb

Using the definition of the proportioning valve’s function and our understanding of system operation, we can use the following rules of thumb:

 

  • On the front to rear split hydraulic systems, a failed proportioning valve should cause both rear wheels to lock up under panic braking – unless one rear brake has a condition that will prevent this (i.e., severely out of adjustment or seized wheel cylinder).
  • On diagonal split hydraulic systems (FWD), it will be infrequent for both proportioning valves to fail simultaneously. If both rear wheels are locking under heavy to panic, look at mechanical causes before condemning the proportioning valves.

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Brake Valve Identification

Brake Valve Identification

by | Jun 28, 2021 | 101 Tech Tips, Brake System Tips

Brake Valve Problem:

Determining whether to add the vehicle’s brake valve(s) to the list of possible causes.

The Cause of Difficulty in Brake Valves:

Brake valves represent the least understood of all the base brake components. Not knowing what the various brakes do, when they do, and how they do it can lead to misdiagnosis.

Brake Valve Solution:

Brake valves are the least understood of all conventional brake components. By understanding what brake valves a vehicle has is essential to the accurate diagnosis of specific brake problems. Because of this, a technician should determine a vehicle’s brake valves by knowing the hydraulic system the car has and looking at the valve itself.

Determining a vehicle’s hydraulic system is very easy on most vehicles. For example, FWD vehicles have diagonal split hydraulic systems (See Figure 47.1). As a result, a diagonal split hydraulic system connects each front wheel with the opposite rear wheel. Likewise, the majority of RWD vehicles have front to rear split hydraulic systems (See Figure 47.2). In a front-to-rear split hydraulic system, the front section is connected hydraulically to one half of the tandem master cylinder while the rear section connects to the other half.

There are some exceptions where RWD vehicles have diagonal split hydraulic systems. Fords and Lincolns are the most common of these exceptions. The easiest way to determine the system is to look at the rear brake lines. For example, if the brake lines from each rear wheel come together into a central line, the vehicle has a front to the rear split hydraulic system. If each rear line continues to the front of the vehicle, the system is a diagonal split hydraulic system.

Schematic 1 - image 47.1

47.1

Schematic 2 - image 47.2

47.2

Looking at Brake Valve

The next step in identifying the type of brake valve(s) a vehicle has involves looking at the brake valve(s). For example, the majority of late-model cars use combination valves. A combination valve is a valve that “combines” more than one valve into a standard housing. Similarly to above, determining the combination valve involves a simple inspection process.

Figure 47.3 shows two typical RWD combination valves. These valves house three separate valves in ordinary housing. The first and easiest valve to identify in this type of valve is the pressure differential switch. The pressure differential switch is in the center of the combination valve (Labeled as “B” in Figure 47.3). The plastic switch identifies it. Any conventional brake valve with this type of switch is a pressure differential switch. For example, the pressure differential switch will complete ground to the red brake warning light in a hydraulic failure.

ABC

47.3

The next valve to determine will be the valve between the master cylinder and rear brakes. To do this, trace the line from the rear brakes to the combination valve. As you find the end of the valve, you’ll see that it houses the proportioning valve. The proportioning valve prevents rear wheel lockup during panic braking. A hydraulic activated valve starts limiting the pressure to the rear brakes at a certain point called the “knee point.” The proportioning valves have the “C” label in Figures 47.3.

Metering Brake Valve

proportioning valve to front brakes

47.4

Finally, the last valve to be identified in a typical combination valve is the metering valve (Labeled as “A” in Figure 47.3). You can locate the metering valve between the master cylinder and front brakes. The majority of RWD vehicles have disc brakes on the front, while drum brakes on the rear have a metering valve. As a result, you should look for the presence of a stem. For example, the stem may protrude from the rubber cap located at the end of the valve, or the rubber cap may cover it up.

The metering valve’s function is to hold off the front brake application until the rear brakes overcome the return springs. The valve does this to allow all brakes to apply at the same time. The combination valve pictured in Figure 47.4 doesn’t have a metering valve. The valve only serves as a junction block for the front brake lines and a proportioning valve.

The next category of brake valves to look at are those combination valves used on FWD vehicles. The majority of brake valves used on FWD vehicles will be dual proportioning valves combined into a standard housing, as shown in Figure 47.5. Each rear brake on a diagonal split hydraulic system is on a separate circuit. Because of the separation, FWD vehicles have two proportioning valves.

Combining the two proportioning valves into standard housing is the most frequently used method. Older FWD vehicles may also include the pressure differential switch in the combination valve, as shown in Figure 47.6. Newer FWD vehicles use a fluid level switch in the master cylinder reservoir instead of the pressure differential switch.

Proportioning Valves

47.5

Pressure Differential Switch

47.6

FWD Combination Brake Valve

An important point to understand about FWD combination valves is that although the lines from the front wheels connect to the valve, fluid flow is unrestricted through the valve. The valve serves only to accomplish the diagonal split “plumbing” but does not act on the fluid flow to the front brakes. FWD vehicles don’t have metering valves or any valve that would limit front braking. The reason for this is due the front brakes do most of the work.

While the majority of FWD vehicles combine the proportioning valves into a familiar housing, others do not. For example, Figures 47.7 & 47.8 show two alternate methods of equipping an FWD vehicle with two proportioning valves. Figure 47.7 shows screw-in proportioning valves used on some four outlet master cylinders, while Figure 47.8 shows side-by-side proportioning valves used on some other FWD vehicles. Some FWD vehicles build the proportioning valves into the master cylinder, as shown in Figure 47.9.

47.7

47.7

47.8

47.8

47.9

47.9

Height Sensitive Brake Valve

The proportioning valves pictured in the previous Figures are hydraulic activated valves. Likewise, they rely on system pressure to know when to operate. However, there is another category of proportioning valve that depends on vehicle ride height to function. Typically, we call these valves height-sensitive control valves, load sensing proportioning valves, or “smart” valves (See page 88 for more detail). The identifying characteristic is the presence of linkage between the valve and the vehicle’s suspension.

For example, there are two typical height-sensitive control valves shown in Figure 47.10. The top valve is from a car equipped with a front to rear split hydraulic system. The bottom valve is from a vehicle equipped with a diagonal split hydraulic system. This valve is a type of combination valve because it houses two separate valves in the same housing.

4.10

47.10

Conventional brake valving may be attached or incorporated into the ABS. The ABS modulator shown in Figure 47.11 has the combination valve attached directly to it. They appear as one unit but are two separate parts. Some ABS systems use screw-in proportioning valves similar to those used in some master cylinders.

Some newer vehicles will not have any mechanical brake valves. Because advances in computer technology have allowed the ABS computer to take over the job of the proportioning valve. These systems let the rear brakes operate up to the point of wheel lock-up and then use micro-pulses to control the pressure to avoid the lock-up. The mico-pulses do not produce the traditional ratcheting pedal drop as experienced during a usual ABS stop. They have tiny and very rapid changes in pressure, which in many cases is unnoticeable in the pedal.

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How to Determine Intermittent Low Brake Pedal on Vehicles equipped with Delco VI ABS

How to Determine Intermittent Low Brake Pedal on Vehicles equipped with Delco VI ABS

by | Jun 25, 2021 | 101 Tech Tips, Brake System Tips

Delco VI ABS

Problem: Intermittent low brake pedal on vehicles equipped with Delco VI ABS (see figure 46.1) 

Cause: EMB (electromagnetic brake) or ESB (expansion spring brake) not holding the motor in homed position. See more info section.

Solution: This condition is usually not straightforward to diagnose. The primary reason for this is its intermittent nature. The most effective method of diagnosing it will be to look for supporting symptoms of the condition. The symptoms that will support the EMB or ESB as the cause of the low pedal will be:

  1. ABS light on with either a code 38 (left front), code 41 (right front), or a code 42 (rear brake circuit).
  2. The problem occurs only ONCE per drive cycle.
  3. The occurrence of the issue should be in time with the ABS light coming on.
  4. There should be an underhood noise associated with the event as the piston causes the gears to the wind.

You can also diagnose it by looking for the symptoms that don’t support the EMB or ESB brakes as being the cause but do support the master cylinder as the cause: 

  1. If you can duplicate the problem with each pump of the brake pedal or more than once without resetting the ABS, the problem is the master cylinder.
  2. If the problem occurs without a code, noise from the modulator/ motor pack, the problem is the master cylinder.
Delco VI motor Driven Pistons
Figure 46.2

More Info

ABS failures rarely cause conventional brake problems, but it is wise to be aware of those that can. The lack of this knowledge can lead to many hours of wasted effort and frustration. While most technicians are familiar with the RWAL/RABS dump valve causing excessive pedal travel, few are aware of a similar problem with the Delco VI. The Delco VI is the most common ABS in use today based on the number of units in the field. It is also unique in how it modulates the brake pressure during an ABS stop. Instead of solenoid sets and a pump motor assembly, the Delco VI uses motor-driven pistons (Figure 46.2).

The pistons are driven up and down by high-speed bidirectional motors to modulate the brake pressure during an ABS stop. During standard braking, the pistons are held in their uppermost or “homed” position by two types of motor brake assemblies. The motor brakes prevent the pressure in the system from pushing the pistons down during standard braking.

Delco VI Expansion Spring Brake
Figure 46.3

The most common type of motor brake is known as the expansion spring brake. As shown in Figure 46.3., this type is located at the base of the motor. The expansion spring prevents the large gear on the base of the piston/worm gear assembly from spinning the small motor gear but allows the motor to drive the small gear in either direction.

Delco VI electromagnetic motor brake

Electromagnetic Motors

Some Delco VI units utilize an electromagnetic motor brake, as shown in Figure 46.4. This design uses three small brake pads in a tripod holder:

  • The holder is attached to the top of the motor shaft.
  • Brake pads clamped between a base plate.
  • And a spring-loaded upper plate. 

This clamping prevents the motor from spinning during standard braking. The system charges an electromagnet during an ABS stop, pulling the top clamping plate up and releasing the tripod assembly. The electromagnet will stay energized during the ABS cycle allowing the motor to drive the piston up and down to regulate pressure.

If any of the three motor brakes fail, it will not maintain the piston in its homed position. The pressure generated from standard braking will drive the piston down in its bore, as shown in Figure 46.5. The extra fluid volume needed to fill this space will cause the brake pedal to travel to the floor, acting as a master cylinder problem.

There will be some noticeable differences when this failure occurs when compared to a bypassing cup seal. The first thing that should happen is the ABS light should come on. The Delco VI will trigger a code 38 for the left front motor brake, code 41 for the right front, and code 42 for the rear. The second thing that you should notice is the problem will happen only once per drive cycle. Once the piston is in the down position and the code has been triggered, the ABS shuts down. The brakes will be regular until the process of the ignition begins again and the vehicle self-tests. When this occurs, the EBCM will drive the pistons to their homed position.

Looking for Symptoms

Newer systems will self-test without driving the vehicle, while older ones require a test drive above four mph. Once the self-test occurs, the problem can reoccur. One last thing that will help differentiate this problem from that of a failed master cylinder. The Delco VI is a loud system when it operates. If a motor brake fails and the pressure pushes down the piston, it will produce a pronounced noise from the modulator/ motor pack assembly. You will hear the piston and motor gear winding. In class, I hot wire the EMB and duplicate this problem to demonstrate it to the students. You can’t miss it. So before you perform some of the involved procedures published, look for these symptoms. Assuming the line lock test is pointing you to the master cylinder/modulator, then in

basic terms:

  • If you can duplicate the problem with each pump of the brake pedal or more than once without resetting the ABS, the problem is the master cylinder.
  • If the problem occurs without a code, noise from the modulator/ motor pack, the problem is the master cylinder.

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