Initial Inspection and Diagnostic Overview
Inspection and Verification1. Most brake concerns that are not related to the electrical or anti-lock system are caused by leaks or mechanical problems in the base brake system.
- For low brake fluid concerns, fill the brake master cylinder reservoir, apply the brakes and inspect for leaks. If no leaks are found, the concern may be caused by caliper piston advance to compensate for disc brake pad wear.
- For mechanical concerns, such as pulling, vibration, drag or squeal, thoroughly inspect the base brake system for wear or damage. Install new as necessary.
- For lack of power assist concerns on E-150, E-250 and E-350, inspect the vacuum system for leaks. For E-450, inspect the power steering pressure supply to the Hydro-Boost. Install new as necessary.
2. To determine if a concern is anti-lock related, disable the anti-lock system by disconnecting the connector on the brake fluid level sensor at the brake master cylinder reservoir. If the concern cannot be duplicated with the anti-lock system disabled, the concern is in the anti-lock system.
3. To test the operation of the brake warning light, do the following:
- Start the engine and observe the brake warning light. The brake warning light should light and then go out.
- With the key to RUN, apply the parking brake. The brake warning light should light and stay lit until the parking brake is released.
4. For premature rear wheel lockup concerns that are not anti-lock related, inspect and adjust the rear brakes. Retest the system.
5. If the concern persists, install a new brake master cylinder fluid control valve mounted in the rear outlet port of the brake master cylinder. Retest the system.
6. For "pedal bottoms out" concerns, check the brake pedal reserve.
- Operate the engine at idle with the transmission in either PARK or NEUTRAL.
- Depress the brake pedal lightly three to four times.
- Allow 15 seconds for vacuum to replenish the power brake booster.
- NOTE: Increased resistance may feel like the brake pedal has bottomed out.
Apply the brake pedal until it stops moving downward or an increased resistance to the brake pedal travel occurs.
- Hold the brake pedal in the applied position and raise the engine speed to approximately 2000 rpm.
- NOTE: The additional movement of the brake pedal is the result of the increased engine manifold vacuum which exerts more force on the power brake booster during engine rundown. This means that additional stroke is available in the brake master cylinder and the brake system is not bottoming out as a customer may believe.
Release the accelerator pedal and shaft and observe that the brake pedal moves downward as the engine returns to idle speed.
For low or spongy brake pedal concerns:
- Check and, if necessary, refill the brake master cylinder reservoir.
- Bleed the brake system and retest the brake pedal feel.
For slow or incomplete brake pedal return concern:
- Inspect for binding, damage, correct installation or interference at the brake pedal.
- Check the power brake booster for binding, damage and correct installation.
For a vibration concern when the brakes are applied on a vehicle with rear drum brakes:
- Road test the vehicle.
- Apply the parking brake and note if vibration is felt.
- If vibration can be felt while applying only the parking brake, machine the brake drums if final internal diameter will be within specification. Retest the vehicle for normal operation.
- If vibration is only felt while applying the brake pedal, use a hub-mount brake lathe to machine the brake discs. Retest the vehicle for normal operation.
Vibration When Brakes Are Applied
For vibration concerns when brakes are applied, perform the following procedure.
Visually inspect:
- tire condition and pressure.
- suspension bushings and ball joints.
Correct as necessary.
1. Verify and isolate the concern. Brake roughness can be felt in:
- the steering wheel.
- the seat.
- the brake pedal.
2. After verifying the concern, check for related concerns in the:
- Technical Service Bulletins.
3. Check wheel bearing end-play and correct as necessary.
4. NOTE: Begin at the front of the vehicle unless the vibration has been isolated to the rear.
Remove the tire and wheel.
5. Remove the brake caliper.
6. Measure and record the brake disc thickness. Install a new brake disc if the thickness after machining will be at or below specification. The specification is molded into the brake disc. Do not machine a new brake disc.
7. For vehicles with a two-piece hub and brake disc assembly:
- Match-mark before disassembly.
- Remove the brake disc.
- Using a die grinder with a mild abrasive (Scotch Brite(R) type), remove any rust or corrosion from the hub and brake disc mounting surfaces.
- Align the match-marks and reinstall the brake disc on the hub.
8. CAUTION: Do not use a bench lathe to machine brake discs.
NOTE: The depth of cut must be between 0.10 and 0.20 mm (0.004 and 0.008 inch). Lighter cuts will cause heat and wear. Heavier cuts will cause poor brake disc surface finish.
Using on-vehicle brake lathe, machine the brake discs. Follow the manufacturer's instructions. After machining, make sure the brake disc still meets the thickness specification.
9. Using a dial indicator, verify that the brake disc lateral runout is now within vehicle specification.
10. Remove the hub adapter.
11. Remove any remaining metal chips from the machining operation.
12. For vehicles with a two-piece hub and brake disc assembly:
- Remove the brake disc from the hub.
- Remove any remaining metal chips from the hub and brake disc mounting surfaces and from the ABS sensor.
- Apply High Temperature Nickel Anti-Seize Lubricant F6AZ-91A94-AA to the mounting surfaces.
- Using the match-marks, mount the brake disc on the hub.
13. Install the caliper and check brake operation.