Lincoln AT4Z-2005-A Booster Assembly - Brake

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Depress brake pedal multiple times with engine off. Check for no change in pedal reserve distance. Depress pedal and start engine. If pedal slightly goes down, operation is normal. Start and turn off engine after 1-2 minutes. Depress brake pedal slowly multiple times. If pedal goes down further first time but gradually rises after second or third depression, booster is airtight. Depress brake pedal while engine is running, then stop engine with pedal depressed. If no change in pedal reserve travel after holding for 30 seconds, booster is airtight. Disassembly requires special tools and is not for home mechanic. Install new or factory rebuilt unit if problem arises. Disconnect brake light switch before disconnecting pedal, booster, or push rod. Pull brake pedal lightly rearward in at-rest position, rotate switch clockwise 45-degrees, and remove. Remove master cylinder. On 2015+ models, remove battery and tray. Disconnect electrical connector from brake vacuum sensor. Remove retaining clip holding hose and wiring harness and move aside. Remove lower instrument panel cover. Remove retaining pin and clip, then disconnect push rod from brake pedal. Note: 2011+ models use one-piece clevis locking pin. Use 12-point, 11 mm wrench or socket to release tabs on pin. Pin is one-time-use and must be replaced if removed. Remove booster mounting nuts from inside, then carefully lift booster unit away from firewall and out of engine compartment. To install booster, reverse removal steps and test brake operation before normal use. When installing brake light switch, insert into bracket and rotate 45-degrees counterclockwise. Be careful not to move brake pedal during installation to avoid switch damage.

Start the engine and turn it off after one or two minutes. Then, depress the brake pedal several times slowly.