WARNING: USE OF ANY OTHER THAN THE APPROVED DOT 3 BRAKE FLUID WILL CAUSE PERMANENT DAMAGE TO BRAKE COMPONENTS AND WILL RENDER THE BRAKES INOPERATIVE.Section 06-00: Brake System, Service | 1996 F-150, F-250, F-350, F-Super Duty Chassis Cab, Motorhome Chassis and Bronco Workshop Manual |
Always check the fluid level in the brake master cylinder reservoir before performing the test procedures.
If the fluid level is not at the MAX line on the brake master cylinder reservoir, add Ford High Performance DOT 3 Brake Fluid C6AZ-19542-AA or DOT 3 equivalent fluid meeting Ford specification ESA-M6C25-A.
WARNING: USE OF ANY OTHER THAN THE APPROVED DOT 3 BRAKE FLUID WILL CAUSE PERMANENT DAMAGE TO BRAKE COMPONENTS AND WILL RENDER THE BRAKES INOPERATIVE.
If a brake is locked and the vehicle must be moved, open a bleeder screw at the locked wheel (1007) to let out enough fluid to relieve the pressure. Close the bleeder screw. This bleeding operation may release the brakes but will not correct the cause of trouble. If this does not relieve the locked wheel condition, service the locked components before proceeding.
Brake Booster Functional Test
Inspect all hoses and connections. All unused vacuum connectors should be capped. Hoses and their connections should be properly secured and in good condition with no holes or collapsed areas.
Brake Booster Operation Check
Check hydraulic brake system for leaks or insufficient fluid.
With transmission (7003) in NEUTRAL, stop engine (6007) and apply parking brake control (2780). Apply brake pedal (2455) several times to exhaust all vacuum in the system.
Apply brake pedal and hold it in the applied position. Start engine. If vacuum system is operating, brake pedal will tend to move downward under constant foot pressure. If no motion is felt, the power brake booster system is not functioning.
Brake Booster
Remove vacuum booster hose (2A047) from power brake booster and check valve connection. Manifold vacuum should be available at the check valve end of the vacuum booster hose with engine at idle speed and transmission in NEUTRAL. If manifold vacuum is available to the power brake booster (2005), connect vacuum booster hose to power brake booster and repeat Steps 2 and 3 above. If no downward movement of brake pedal is felt, replace power brake booster.
Operate engine a minimum of 10 seconds at fast idle. Stop engine, and let vehicle stand for 10 minutes. Then, apply brake pedal with approximately 89 N (20 lbs.) of force. Brake pedal feel should be the same as that noted with engine operating. If brake pedal feels hard (no power assist), replace check valve and retest. If brake pedal feels spongy, bleed hydraulic system to remove air. Refer to Section 06-06.
Normal Hydro-Boost will produce certain characteristic noises that current technology cannot correct. These noises occur, for the most part, when the brake pedal is manipulated in a manner not associated with the everyday braking habits. The general categories of normal operating noise are hissing noise and clunk, click and chatter noises.
The Hydro-Boost will emit normal hissing noises when above normal pedal efforts are applied. The hiss sounds are particularly noticeable with the vehicle motionless and will increase in intensity as pedal efforts increase and system operating temperatures increase. Loud hissing sounds at or below normal pedal effort (111-156 N [25-35 lbs.]) warrant investigation and possible correction.
Clunk, chatter or clicking noises will be heard when the brake pedal is quickly released from hard pedal efforts. Due to "quick release for hard application" requirement for the noises, they will most likely be heard with the vehicle motionless or during a fast stop-start condition. Refer to Pinpoint Test P and Pinpoint Test Q for Hydro-Boost Power Brake Booster Diagnosis.
Possible sources of leakage from the Hydro-Boost power brake booster are:
| Item | Part Number | Description |
|---|---|---|
| 1 | — | Spool Plug Seal (Part of 2005) |
| 2 | — | Return Port to Reservoir (Part of 2005) |
| 3 | — | Inlet Port from Power Steering Pump (Part of 2005) |
| 4 | — | Outlet Port to Steering Gear (Part of 2005) |
| 5 | — | Input Seal (Part of 2005) |
| 6 | — | Housing Seal (Part of 2005) |
| 7 | — | Accumulator Seal (Part of 2005) |
| 8 | — | Piston Seal (Part of 2005) |
| 9 | 2005 | Power Brake Booster |
Check Valve
The function of the power brake booster check valve (2365) is to allow manifold vacuum to enter the power brake booster and prevent the escape of vacuum in case manifold vacuum is lost during sustained full throttle operation.
To test the functioning of the power brake booster check valve, perform the following:
Vacuum System Test
To test the vacuum system, refer to Pinpoint Test N in this section.
Brake Master Cylinder
Normal Conditions
The following conditions are considered normal and are not indications that the brake master cylinder is in need of service.
Condition 1: During normal operation of the master cylinder, the fluid level in the brake master cylinder reservoir will rise during brake application and fall during release. The net fluid level, i.e., after brake application and release, will remain unchanged.
Condition 2: A trace of brake fluid will exist on the booster shell below the master cylinder mounting flange. This results from the normal lubricating action of the master cylinder bore and seal.
Condition 3: Fluid level will decrease with pad wear.
Abnormal Conditions
Changes in brake pedal feel or travel are indicators that something could be wrong in the brake system. Refer to Symptom Chart for abnormal condition diagnosis.
Bypass Condition Test
For the bypass condition test, refer to Pinpoint Test M in this section.
Non-Pressure Leaks
The only part of the brake system that could have a brake fluid loss that does not appear when the system is under pressure is the brake master cylinder reservoir area. Brake fluid loss could be caused by any of the following:
The brake master cylinder reservoir grommets are not separately serviceable and must be replaced with a new brake master cylinder reservoir.
NOTE: The wet appearance of the outer cover of rubber brake hoses is called sweating and is normal with neoprene rayon braid hose. It is not evidence of a brake fluid leak and will not result in a loss of pressure in the system.
Replace a flexible brake hose if it shows signs of softening, cracking or other damage.When installing a new brake hose, position the hose to avoid contact with other vehicle parts.Compensator Port Check
The purpose of the compensator ports in the brake master cylinder is to supply any additional brake fluid required by the system due to brake pad wear and to allow brake fluid returning from the brake lines to the brake master cylinder to enter the brake master cylinder reservoir.
The returning brake fluid will cause a slight turbulence in the brake master cylinder reservoir. If turbulence is seen in the brake master cylinder reservoir upon release of the brake pedal, this is normal and shows that the compensating ports are not plugged.
Brake Master Cylinder Fluid Control Valve
If there is a brake system concern such as premature rear brake lock up that cannot be attributed to the rear anti-lock brake system (RABS), the cause may be a malfunctioning brake master cylinder fluid control valve (2C161).
Check the brake master cylinder fluid control valve by substituting the suspected bad valve with a known good one. If the condition persists, recheck the RABS. Refer to Section 06-06 for brake master cylinder fluid control valve replacement procedure. Refer to Section 06-09A for RABS diagnostics.