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ABS Safety Precautions

ABS Safety Precautions

Problem: Potential safety risk and possible component damage

Cause: Not adhering to proper safety precautions when servicing ABS equipped



• NEVER open a bleeder valve or loosen a hydraulic line while ABS is pressurized. The accumulator must be depressurized in accordance with individual manufacturer’s repair instructions.

• NEVER disconnect or reconnect any electrical connectors while ignition is on. Damage to ABS control unit may result.

• ONLY use specially designed brake hoses/lines on ABS equipped vehicles.

• DO NOT tap on speed sensor components (sensor, sensor rings). Speed rings must be pressed onto hubs, NOT hammered onto hubs. Striking these components can cause demagnetization or polarization, affecting accuracy of speed signal returning to ABS control unit.

• DO NOT mix tire sizes. Increasing the width, as long as tires remain close to the original diameter, is acceptable. Rolling diameter must be identical for all 4 tires. Some manufacturers recommend tires of the same brand, style and type. Failure to follow this precaution may cause inaccurate wheel speed readings.

• DO NOT over tighten wheel lug nuts. Bent rotor or brake drum may occur, which can lead to inaccurate wheel speed readings.

• DO NOT contaminate speed sensor components with grease. Only use recommended coating when system calls for an anticorrosion coating.

• When speed sensor components have been removed, ALWAYS check sensor-to-ring air gaps when applicable. These specifications can be found in each appropriate article.

• ONLY use recommended brake fluids. DO NOT use silicone brake fluids in an ABS equipped vehicle.

• When installing transmitting devices (CB’s, bluetooth, etc.) on ABS equipped vehicles, DO NOT locate the antenna near the ABS control unit (or any control unit).

• Disconnect all on-board computers when using electric welding equipment. Alternate method: Place welding clamp as close to work as possible.

• DO NOT expose the ABS control unit to prolonged periods of high heat (185‘F/85‘C for 2 hours is generally considered a maximum limit).

• Do not disconnect the battery to clear codes. This only works on a small number of vehicles.

• Do not overfill the brake fluid reservoir. Follow manufacturers instructions.

Vacuum Booster – Partial Brake Assist Diagnostic Tip

Vacuum Booster – Partial Brake Assist Diagnostic Tip

Problem: Poor brake assist – lack of stopping power or having to depress the brake pedal harder than usual to stop.

Solution: If you suspect the brake vacuum booster of providing only partial assist use the following steps to determine if it is a vacuum supply problem or a booster problem.


  1. Pull another vehicle of similar engine size next to the vehicle in question and connect that vehicle’s vacuum source to the suspect booster. I usually use an old piece of air line for this purpose.
  2. With the new source vehicle running apply the brake pedal in the suspect vehicle and check for a change in brake assist. If no change is felt then the booster is likely the culprit. If there is a change in the assist (pedal feels normal) then the vacuum plumbing is likely the problem.
  3. If the vacuum plumbing is the problem then closely inspect the components between the booster and the vacuum source to locate the problem.

More info: The vacuum booster has proven to be one of the most reliable parts in the brake system. This reliability has caused many technicians to overlook the booster as a potential source of brake problems. The booster is also high on the list of unpleasant parts to install. With this in mind it is a good idea to make sure the booster is the source of the problem before installing another unit.

Unfortunately this article does not allow a full explanation of the inner workings of a booster but you do need a basic idea of what is going on inside to allow accurate diagnosis. The main components inside a booster that control the assist function are the air and vacuum valves. The position of the valves determine what stage the booster is in. There are three stages of booster operation – unapplied pedal (no assist), applied pedal (assist) and hold (isolate assist). When a vehicle is running and the brake pedal is not applied the booster is in a suspended state. The vacuum valve is open and the air valve is closed causing the diaphragm to be “suspended”. When the brake is applied the vacuum valve closes and the air valve opens. Closing the vacuum valve isolates the boost chamber from the vacuum chamber. Opening the air valve allows atmospheric air to enter the boost chamber. The difference in pressures on either side of the diaphragm is what produces the assist. When the driver is at a stop light the pedal pressure is usually reduced causing the booster to go into the hold mode. During the hold mode the air valve closes and the vacuum valve remains closed. This isolates the air in the boost chamber until the driver releases the brake pedal.

Pressure Testing Brake Hydraulic System Tips

Pressure Testing Brake Hydraulic System Tips

To properly use the results of a pressure test it is best to first understand the pressures generated during different types of braking and the function of the different brake valves used in the system.

You will need to obtain a brake hydraulic pressure test kit similar to the image above

Because most manufacturers do not publish pressure specifications it is often necessary to use generic values. Typically the hydraulic pressure generated during light to normal braking will be from 300psi to 500psi. Panic braking is generally considered to start at about 600psi. The proportioning valve will start to limit pressure to the rear brakes at this time. If a full panic stop is taking place the proportioning valve will limit the rear brake pressure to about 800psi while the front brakes will climb to between 1500psi to 2000psi. The psi numbers presented here are averages based on many tests performed on different vehicles.

Two types of pressure testing are common, checking the front to rear brake pressures and checking an axleʼs side to side pressures. The uses of each of the tests is explained below:

Front to Rear pressure for a properly operating system

Checking front to rear pressure:

Performing a front to rear pressure test can be done to help diagnose the following conditions:

  • Proportioning valve malfunction – Diagnose the cause of rear wheel lockup during panic braking or possibly a proportioning valve causing a restriction (wonʼt reopen after release).
  • Premature front brake pad wear – Pressure tests are used to determine if a lack of rear braking is related to the cause of front pad wear.
  • Height sensitive control valve diagnosis or adjustment – These valves usually incorporate pressure tests in the adjustment procedure and can be diagnosed using pressure gauges.

Side to Side pressure for a malfunctioning system

Checking side to side pressure:

Performing a side to side pressure test can be done to help diagnose the following conditions:

  • Brake pull – different hydraulic pressures results in different clamping forces and can result in a pull
  • Side to side pad wear differences – if the pads on one side of an axle are wearing faster than the pads on the opposite side

Performing a Front to Rear Pressure Test

  1. Connect the gauges using the manufacturerʼs instructions. Most pressure gauges connect to the caliper or wheel cylinderʼs bleeder screw hole (See Figure 85.4). Make sure to properly bleed each gauge hose.
  2. Place the gaugeʼs memory needles on zero.
  3. Start the vehicle. Apply the brake first using light to moderate force as if making a normal stop. Note the pressure readings, under this pedal force the front pressures should not be over 500psi in most vehicles. Compare the front and rear pressure readings. At this pedal force they should be equal or near equal. If equal continue to next step. If there is a large difference with the rears being substantially lower than the fronts there is a potential problem. The list of possible causes will depend on the system configuration. The list will include:
  • Restricted proportioning valve
  • Restricted brake hose to the wheel or axle
  • Restricted steel brake line
  • Out of adjustment or restricted height sensitive control valve
  • Restriction in ABS modulator


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