Mopar 4721721AA Booster-Power Brake

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To perform an operating check on the brake system, start by depressing the pedal and starting the engine; a slight pedal depression is normal. Depress the brake pedal multiple times with the engine running and ensure there's no change in pedal reserve distance. For an airtightness check, start the engine and turn it off after a minute or two, then depress the brake pedal slowly. If the pedal initially goes down farther but gradually rises with subsequent depressions, the booster is airtight. Additionally, depress the brake pedal while the engine is running, then stop the engine while holding the pedal down; if there's no change in pedal reserve travel after 30 seconds, the booster is airtight. As for removal, the power brake booster unit doesn't require special maintenance and should never be disassembled; if an issue arises, it must be replaced. To start, remove vacuum from the booster by pumping the pedal with the engine off until it feels firm. Then, remove the battery and battery tray, and if applicable, detach the cruise control servo unit bracket. Remove the wiper motor and linkage assembly, and clean the area where the master cylinder attaches to the booster. Detach the master cylinder from the booster while keeping it supported and disconnect the vacuum hose from the check valve on the outside of the booster. Remove the knee bolster from the driver's side of the instrument panel. Under the dash, disconnect the brake pedal pushrod and replace the retaining clip for safety. Remove the nuts securing the booster to the firewall, and from the engine compartment, withdraw the booster unit. To install the booster, position it on the firewall and tighten the retaining nuts. Reconnect the brake pedal to the booster pushrod with a new retainer clip, and perform the remaining installation steps in reverse, ensuring a new vacuum seal is installed on the master cylinder before reattaching it to the booster. Finally, test the brake system's operation carefully before normal vehicle operation.

Start the engine and turn it off after a minute or two, then depress the brake pedal slowly. If the pedal initially goes down farther but gradually rises with subsequent depressions, the booster is airtight. Additionally, depress the brake pedal while the engine is running, then stop the engine while holding the pedal down. If there's no change in pedal reserve travel after 30 seconds, the booster is airtight.

Power brake booster units should not be disassembled, as they require special tools and are fairly complex. It is best to replace a defective booster unit with a new or rebuilt one.