Initial Inspection and Diagnostic Overview

E250, E350, E Super Duty.
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. Repair as necessary.
- For lack of power assist concerns on, inspect the vacuum system for leaks. Repair as necessary. Refer to Hydraulic System.
2. To determine if the 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. For diagnosis of anti-lock concerns, refer to Antilock Brake System and begin diagnosis with the System Pre-Check. Testing and Inspection
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 in "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, replace the brake master cylinder fluid control valve mounted in the rear outlet port of the brake master cylinder. Retest the system.
5. 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.
- Apply the brake pedal until it stops moving downward or an increased resistance to the brake pedal travel occurs.

NOTE: Increased resistance may feel like the brake pedal has bottomed out.

- Hold the brake pedal in the applied position and raise the engine speed to approximately 2000 rpm.

NOTE: Increased resistance may feel like the brake pedal has bottomed out.

- Release the accelerator pedal and shaft and observe that the brake pedal moves downward as the engine returns to idle speed.

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.