ABS Hydraulic Modulator/Master Cylinder Assembly
- Disconnect the negative battery cable.
- Disengage the two solenoid electrical connectors and the fluid level sensor connector.
- Detach the 6-pin and 3-pin motor pack electrical connectors.
- Wrap a shop towel around the hydraulic brake lines, then disconnect the four brake lines from the modulator.
Cap the disconnected lines to prevent the loss of fluid and the entry of moisture and contaminants.
- Unfasten the 2 nuts attaching the ABS hydraulic modulator/master cylinder assembly to the vacuum booster.
- Remove the ABS hydraulic modulator assembly from the vehicle.
- Install the ABS hydraulic modulator assembly to the vehicle. Secure using the two attaching nuts and tighten to 20 ft. lbs. (27 Nm).
- Uncap and connect the 4 brake pipes to the modulator assembly. Tighten to 13 ft. lbs. (17 Nm).
- Attach the 6-pin and 3-pin electrical connectors.
- Engage the fluid level sensor connector and the two solenoid electrical connections.
- Properly bleed the ABS system, as outlined later in this section.
- Connect the negative battery cable.
Bleeding the ABS System
SYSTEM FILLING
The master cylinder reservoirs must be kept properly filled to prevent air from entering the system. No special filling procedures are required because of the anti-lock system.
When adding fluid, use only DOT 3 fluid; the use of DOT 5 or silicone fluids is specifically prohibited. Use of improper or contaminated fluid may cause the fluid to boil or cause the rubber components in the system to deteriorate. Never use any fluid with a petroleum base or any fluid which has been exposed to water or moisture.
BLEEDING THE ABS HYDRAULIC SYSTEMBefore bleeding the ABS brake system, the front and rear displacement cylinder pistons must be returned to the topmost position. The preferred method uses a Tech 1® or T-100® scan tool to perform the rehoming procedure. If a Tech 1® is not available, the second procedure may be used, but it must be followed EXACTLY.
Rehome ProcedureWITH TECH 1® OR T-100® (PREFERRED METHOD)
- Using a Tech 1® or T-100® (CAMS), select F5: Motor Rehome. The motor rehome function cannot be performed if current DTC's are present. If DTC's are present, the vehicle must be repaired and the codes cleared before performing the motor rehome function.
- The entire brake system should now be bled using the pressure or manual bleeding procedures outlined later in this section.
Do not place your foot on the brake pedal through this entire procedure unless specifically instructed to do so.
This method can only be used if the ABS warning lamp is not illuminated and not DTC's are present.
- Remove your foot from the brake pedal.
- Start the engine and allow it to run for at least 10 seconds while observing the ABS warning lamp.
- If the ABS warning lamp turned ON and stayed ON after about 10 seconds, the bleeding procedure must be stopped and a Tech 1® must be used to diagnose the ABS function.
- If the ABS warning lamp turned ON for about 3 seconds, then turned OFF and stayed OFF, turn the ignition OFF .
- Repeat Steps 1-4 one more time.
- The entire brake system should now be bled by following the guide or pressure bleeding procedure.
See Figures 1 and 2
The pressure bleeding equipment must be of the diaphragm type. It must have a rubber diaphragm between the air supply and the brake fluid to prevent air, moisture and other contaminants from entering the hydraulic system.
- Clean the master cylinder fluid reservoir cover and surrounding area, then remove the cover.
- Add fluid, if necessary to obtain a proper fluid level.
- Connect bleeder adapter J 35589, or equivalent, to the brake fluid reservoir, then connect the bleeder adapter to the pressure bleeding equipment.
- Adjust the pressure bleed equipment t o 5-10 psi (35-70 kPa) and wait about 30 seconds to be sure there is no leakage.
- Adjust the pressure bleed equipment to 30-35 psi (205-240 kPa).
- With the pressure bleeding equipment connected and pressurized, proceed as follows:
- Attach a clear plastic bleeder hose to the rearward bleeder valve on the hydraulic modulator.
- Slowly open the bleeder valve and allow fluid to flow until no air is seen in the fluid.
- Close the valve when fluid flows out without any air bubbles.
- Repeat Steps 6b and 6c until no air bubbles are present.
- Relocate the bleeder hose on the forward hydraulic modulator bleed valve and repeat Steps 6a through 6d.
Fig. Fig. 1: ABS pressure bleeding components
- Tighten the bleeder valve to 80 inch lbs. (9 Nm).
- Proceed to bleed the hydraulic modulator brake pipe connections as follows with the pressure bleeding equipment connected and pressurized:
- Slowly open the forward brake pipe tube nut on the hydraulic modulator and check for air in the escaping fluid.
- When the air flow ceases, immediately tighten the tube nut. Tighten the tube nut to 18 ft. lbs. (24 Nm).
- Repeat Steps 8a and 8b for the remaining three brake pipe connections moving from the front to the rear.
Fig. Fig. 2: Position a shop rag to catch escaping brake fluid
- Raise and safely support the vehicle.
- Proceed, as outlined in the following steps, to bleed the wheel brakes in the following sequence: right rear, left rear, right front, then left front.
- Attach a clear plastic bleeder hose to the bleeder valve at the wheel, then submerge the opposite hose end in a clean container partially filled with clean brake fluid.
- Slowly open the bleeder valve and allow the fluid to flow.
- Close the valve when fluid begins to flow without any air bubbles. Tap lightly on the caliper or backing plate to dislodge any trapped air bubbles.
- Repeat Step 11 on the other brakes using the earlier sequence.
- Remove the pressure bleeding equipment, including bleeder adapter J 35589.
- Carefully lower the vehicle, then check the brake fluid and add if necessary. Don't forget to put the reservoir cap back on.
- With the ignition turned to the RUN position, apply the brake pedal with moderate force and hold it. Note the pedal travel and feel. If the pedal feels firm and constant and the pedal travel is not excessive, start the engine. With the engine running, recheck the pedal travel. If it's still firm and constant and pedal travel is not excessive, go to Step 17.
- If the pedal feels soft or has excessive travel either initially or after the engine is started, the following procedure may be used:
- With the Tech 1® scan tool, release then apply each motor 2-3 times and cycle each solenoid 5-10 times. When finished, be sure to apply the front and rear motors to ensure the pistons are in the upmost position. DO NOT DRIVE THE VEHICLE.
- If a Tech 1® is not available, remove your foot from the brake pedal, start the engine and allow it run for at least 10 seconds to initialize the ABS. DO NOT DRIVE THE VEHICLE. After 10 seconds, turn the ignition OFF . The initialization procedure most be repeated 5 times to ensure any trapped air has been dislodged.
- Repeat the bleeding procedure, starting with Step 1.
- Road test the vehicle, and make sure the brakes are operating properly.
See Figure 3
- Clean the master cylinder fluid reservoir cover and surrounding area, then remove the cover.
- Add fluid, if necessary to obtain a proper fluid level, then put the reservoir cover back on.
- Prime the ABS hydraulic modulator/master cylinder assembly as follows:
- Attach a bleeder hose to the rearward bleeder valve, then submerge the opposite hose end in a clean container partially filled with clean brake fluid.
- Slowly open the rearward bleeder valve.
- Depress and hold the brake pedal until the fluid begins to flow.
- Close the valve, then release the brake pedal.
- Repeat Steps 3b-3d until no air bubbles are present.
- Relocate the bleeder hose to the forward hydraulic modulator bleeder valve, then repeat Steps 3a-3e.
- Once the fluid is seen to flow from both modulator bleeder valves, the ABS modulator/master cylinder assembly is sufficiently full of fluid. However, it may not be completely purged of air. At this point, move to the wheel brakes and bleed them. This ensures that the lowest points in the system are completely free of air and then the assembly can purged of any remaining air.
Fig. Fig. 3: ABS hydraulic modulator/master cylinder bleeder locations
- Remove the fluid reservoir cover. Fill to the correct level, if necessary, then fasten the cover.
- Raise and safely support the vehicle.
- Proceed, as outlined in the following steps, to bleed the wheel brakes in the following sequence: right rear, left rear, right front, then left front.
- Attach a clear plastic bleeder hose to the bleeder valve at the wheel, then submerge the opposite hose end in a clean container partially filled with clean brake fluid.
- Open the bleeder valve.
- Have an assistant slowly depress the brake pedal.
- Close the valve and slowly release the release the brake pedal.
- Wait 5 seconds.
- Repeat Steps 7a-7e until the brake pedal feels firm at half travel and no air bubbles are observed in the bleeder hose. To assist in freeing the entrapped air, tap lightly on the caliper or braking plate to dislodge any trapped air bubbles.
- Repeat Step 7 for the remaining brakes in the sequence given earlier.
- Carefully lower the vehicle.
- Remove the reservoir cover, then fill to the correct level with brake fluid and replace the cap.
- Bleed the ABS hydraulic modulator/master cylinder assembly as follows:
- Attach a clear plastic bleeder hose to the rearward bleeder valve on the modulator, then submerge the opposite hose end in a clean container partially filled with clean brake fluid.
- Have an assistant depress the brake pedal with moderate force.
- Slowly open the rearward bleeder valve and allow the fluid to flow.
- Close the valve, then release the brake pedal.
- Wait 5 seconds.
- Repeat Steps 11a-11e until no air bubbles are present.
- Relocate the bleeder hose to the forward hydraulic modulator bleeder valve, then repeat Steps 11a-11f.
- Carefully lower the vehicle, then check the brake fluid and add if necessary. Don't forget to put the reservoir cap back on.
- With the ignition turned to the RUN position, apply the brake pedal with moderate force and hold it. Note the pedal travel and feel. If the pedal feels firm and constant and the pedal travel is not excessive, start the engine. With the engine running, recheck the pedal travel. If it's still firm and constant and pedal travel is not excessive, road test the vehicle and make sure the brakes are operating properly.
- If the pedal feels soft or has excessive travel either initially or after the engine is started, the following procedure may be used:
- With the Tech 1® scan tool, Release then Apply each motor 2-3 times and cycle each solenoid 5-10 times. When finished, be sure to Apply the front and rear motors to ensure the pistons are in the upmost position. DO NOT DRIVE THE VEHICLE.
- If a Tech 1® scan tool is not available, remove your foot from the brake pedal, start the engine and allow it run for at least 10 seconds to initialize the ABS. DO NOT DRIVE THE VEHICLE. After 10 seconds, turn the ignition OFF . The initialization procedure most be repeated 5 times to ensure any trapped air has been dislodged.
- Repeat the bleeding procedure, starting with Step 1.
- Road test the vehicle, and make sure the brakes are operating properly.
Brake Fluid Level Switch
REMOVAL & INSTALLATION
See Figures 1 and 2
- Disconnect the negative battery cable.
- Detach the electrical connector from the fluid level switch.
- Using needle-nose pliers, compress the switch locking tabs at the inboard side of the master cylinder reservoir and remove the switch.
Fig. Fig. 1: The fluid level switch is mounted in the master cylinder reservoir
Fig. Fig. 2: Remove the switch by using needle-nose pliers to compress the switch locking tabs
- Press the fluid level switch into the master cylinder reservoir until it snaps into place.
- Attach the switch electrical connector.
- Connect the negative battery cable.
The Anti-lock Braking System (ABS) is standard equipment on all J-body cars covered by this guide. ABS provides the driver with 3 important benefits over standard braking systems: increased vehicle stability, improved vehicle steerability, and potentially reduced stopping distances during braking. It should be noted that although the ABS-VI system offers definite advantages, the system cannot increase brake pressure above master cylinder pressure applied by the driver and cannot apply the brakes itself.
The ABS-VI Anti-lock Braking System consists of a conventional braking system with vacuum power booster, compact master cylinder, front disc brakes, rear drum brakes and interconnecting hydraulic brake lines augmented with the ABS components. The ABS-VI system includes a hydraulic modulator assembly, Electronic Brake Control Module (EBCM) or Electronic Brake and Traction Control Module (EBTCM) (these are all different Computer Control Modules which differ depending upon vehicle year and application), a brake fluid level sensor, a system relay, 4 wheel speed sensors, interconnecting wiring and an amber ABS warning light.
The EBCM/EBTCM monitors inputs from the individual wheel speed sensors and determines when a wheel or wheels is/are about to lock up. The EBCM/EBTCM controls the motors on the hydraulic modulator assembly to reduce brake pressure to the wheel about to lock up. When the wheel regains traction, the brake pressure is increased until the wheel again approaches lock-up. The cycle repeats until either the vehicle comes to a stop, the brake pedal is released, or no wheels are about to lock up. The EBCM/EBTCM also has the ability to monitor itself and can store diagnostic codes in a non-volatile (will not be erased if the battery is disconnected) memory. The EBCM/EBTCM is serviced as an assembly.
The ABS-VI braking system employs 2 modes: base (conventional) braking and anti-lock braking. Under normal braking, the conventional part of the system stops the vehicle. When in the ABS mode, the Electromagnetic Brakes (EMB) action of the ABS system controls the two front wheels individually and the rear wheels together. If the one rear wheel is about to lock up, the hydraulic pressure to both wheels is reduced, controlling both wheels together.
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Hydraulic systems are used to actuate the brakes of all modern automobiles. The system transports the power required to force the frictional surfaces of the braking system together from the pedal to the individual brake units at each wheel. A hydraulic system is used for two reasons.
First, fluid under pressure can be carried to all parts of an automobile by small pipes and flexible hoses without taking up a significant amount of room or posing routing problems.
Second, a great mechanical advantage can be given to the brake pedal end of the system, and the foot pressure required to actuate the brakes can be reduced by making the surface area of the master cylinder pistons smaller than that of any of the pistons in the wheel cylinders or calipers.
The master cylinder consists of a fluid reservoir along with a double cylinder and piston assembly. Double type master cylinders are designed to separate the front and rear braking systems hydraulically in case of a leak. The master cylinder coverts mechanical motion from the pedal into hydraulic pressure within the lines. This pressure is translated back into mechanical motion at the wheels by either the wheel cylinder (drum brakes) or the caliper (disc brakes).
Steel lines carry the brake fluid to a point on the vehicle's frame near each of the vehicle's wheels. The fluid is then carried to the calipers and wheel cylinders by flexible tubes in order to allow for suspension and steering movements.
In drum brake systems, each wheel cylinder contains two pistons, one at either end, which push outward in opposite directions and force the brake shoe into contact with the drum.
In disc brake systems, the cylinders are part of the calipers. At least one cylinder in each caliper is used to force the brake pads against the disc.
All pistons employ some type of seal, usually made of rubber, to minimize fluid leakage. A rubber dust boot seals the outer end of the cylinder against dust and dirt. The boot fits around the outer end of the piston on disc brake calipers, and around the brake actuating rod on wheel cylinders.
The hydraulic system operates as follows: When at rest, the entire system, from the piston(s) in the master cylinder to those in the wheel cylinders or calipers, is full of brake fluid. Upon application of the brake pedal, fluid trapped in front of the master cylinder piston(s) is forced through the lines to the wheel cylinders. Here, it forces the pistons outward, in the case of drum brakes, and inward toward the disc, in the case of disc brakes. The motion of the pistons is opposed by return springs mounted outside the cylinders in drum brakes, and by spring seals, in disc brakes.
Upon release of the brake pedal, a spring located inside the master cylinder immediately returns the master cylinder pistons to the normal position. The pistons contain check valves and the master cylinder has compensating ports drilled in it. These are uncovered as the pistons reach their normal position. The piston check valves allow fluid to flow toward the wheel cylinders or calipers as the pistons withdraw. Then, as the return springs force the brake pads or shoes into the released position, the excess fluid reservoir through the compensating ports. It is during the time the pedal is in the released position that any fluid that has leaked out of the system will be replaced through the compensating ports.
Dual circuit master cylinders employ two pistons, located one behind the other, in the same cylinder. The primary piston is actuated directly by mechanical linkage from the brake pedal through the power booster. The secondary piston is actuated by fluid trapped between the two pistons. If a leak develops in front of the secondary piston, it moves forward until it bottoms against the front of the master cylinder, and the fluid trapped between the pistons will operate the rear brakes. If the rear brakes develop a leak, the primary piston will move forward until direct contact with the secondary piston takes place, and it will force the secondary piston to actuate the front brakes. In either case, the brake pedal moves farther when the brakes are applied, and less braking power is available.
All dual circuit systems use a switch to warn the driver when only half of the brake system is operational. This switch is usually located in a valve body which is mounted on the firewall or the frame below the master cylinder. A hydraulic piston receives pressure from both circuits, each circuit's pressure being applied to one end of the piston. When the pressures are in balance, the piston remains stationary. When one circuit has a leak, however, the greater pressure in that circuit during application of the brakes will push the piston to one side, closing the switch and activating the brake warning light.
In disc brake systems, this valve body also contains a metering valve and, in some cases, a proportioning valve. The metering valve keeps pressure from traveling to the disc brakes on the front wheels until the brake shoes on the rear wheels have contacted the drums, ensuring that the front brakes will never be used alone. The proportioning valve controls the pressure to the rear brakes to lessen the chance of rear wheel lock-up during very hard braking.
Warning lights may be tested by depressing the brake pedal and holding it while opening one of the wheel cylinder bleeder screws. If this does not cause the light to go on, substitute a new lamp, make continuity checks, and, finally, replace the switch as necessary.
The hydraulic system may be checked for leaks by applying pressure to the pedal gradually and steadily. If the pedal sinks very slowly to the floor, the system has a leak. This is not to be confused with a springy or spongy feel due to the compression of air within the lines. If the system leaks, there will be a gradual change in the position of the pedal with a constant pressure.
Check for leaks along all lines and at wheel cylinders. If no external leaks are apparent, the problem is inside the master cylinder.
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POWER BRAKE BOOSTERS
Virtually all modern vehicles use a vacuum assisted power brake system to multiply the braking force and reduce pedal effort. Since vacuum is always available when the engine is operating, the system is simple and efficient. A vacuum diaphragm is located on the front of the master cylinder and assists the driver in applying the brakes, reducing both the effort and travel he must put into moving the brake pedal.
The vacuum diaphragm housing is normally connected to the intake manifold by a vacuum hose. A check valve is placed at the point where the hose enters the diaphragm housing, so that during periods of low manifold vacuum brakes assist will not be lost.
Depressing the brake pedal closes off the vacuum source and allows atmospheric pressure to enter on one side of the diaphragm. This causes the master cylinder pistons to move and apply the brakes. When the brake pedal is released, vacuum is applied to both sides of the diaphragm and springs return the diaphragm and master cylinder pistons to the released position.
If the vacuum supply fails, the brake pedal rod will contact the end of the master cylinder actuator rod and the system will apply the brakes without any power assistance. The driver will notice that much higher pedal effort is needed to stop the car and that the pedal feels harder than usual.
Vacuum Leak Test- Operate the engine at idle without touching the brake pedal for at least one minute.
- Turn off the engine and wait one minute.
- Test for the presence of assist vacuum by depressing the brake pedal and releasing it several times. If vacuum is present in the system, light application will produce less and less pedal travel. If there is no vacuum, air is leaking into the system.
- With the engine OFF , pump the brake pedal until the supply vacuum is entirely gone.
- Put light, steady pressure on the brake pedal.
- Start the engine and let it idle. If the system is operating correctly, the brake pedal should fall toward the floor if the constant pressure is maintained.
Power brake systems may be tested for hydraulic leaks just as ordinary systems are tested.
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