Troubleshooting a Power Brake for an F250by William Machin
The power brake for a Ford F250 operates with a vacuum booster. The booster mounts at the firewall inside the engine compartment and a booster rod connects to the upper end of the brake pedal inside the cab. Vacuum is provided by a hose that connects to the booster and the engine's intake manifold. Pushing the brake pedal when the engine is running compresses a diaphragm in the booster, providing power assist. The booster needs a vacuum supply from the engine and a good diaphragm to work properly. You can troubleshoot the power brake on a Ford F250 using a simple procedure.
Raise the engine hood and locate the vacuum booster at the firewall on the driver's side. Depending on the year of the F250, a master cylinder mounts on the front or back of the booster.
Start the engine. Locate the vacuum hose that attaches at the front of the vacuum booster. Twist the hose by hand as you pull it off the plastic nozzle.
Put a finger over the end of the vacuum hose. You should feel strong vacuum suction. In the event the suction is not substantial, check the vacuum by holding a vacuum gauge securely at the end of the hose. A reading below 16 inches of vacuum indicates a hose leak or a blockage in the vacuum system within the engine.
Remove the vacuum hose from the brass nozzle on the intake manifold, using a screwdriver to loosen the hose clamp counterclockwise. Twist the hose by hand and pull it off the nozzle. Inspect the hose for cracks by flexing it at points along the hose. A bad hose must be replaced. Proceed to the next step before you install the new hose or reconnect the existing hose.
Position an open-end wrench on the flats at the base of the booster nozzle. Remove the hose nozzle by turning it counterclockwise. The nozzle has a check valve that traps vacuum in the booster. Blow through each end of the nozzle. Air should pass one way only. The nozzle and check valve must be replaced when air passes through the nozzle both ways.
Sit in the driver's seat with the engine off. Press the brake pedal down and start the engine. You should feel the pedal depress slightly as engine vacuum enters the booster and flexes the diaphragm.
Turn the engine off. Lift your foot off the brake pedal. With a good vacuum hose and check valve, the system retains vacuum, allowing one or two power-assist stops until the engine is restarted.
Press the brake pedal again slowly with the engine off. You should feel good pressure on the pedal. In the event the pedal is mushy or depresses beyond the halfway point, the booster diaphragm is faulty and the booster needs to be replaced.
- Most auto parts and auto accessory stores carry inexpensive vacuum gauges.
Items you will need
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