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 engine and turn off 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. If problem arises, install new or factory rebuilt unit. In engine compartment, remove nuts attaching master cylinder to booster and carefully pull forward until it clears mounting studs. Be careful not to bend or kink
Brake Lines. On 2005+ Expedition/Navigator models, remove air filter housing and unbolt and set aside degas bottle. Disconnect vacuum hose from power brake booster. Before removing booster, detach intake manifold vacuum hose from booster housing. In passenger compartment, remove cotter pin and
Brake Light Switch and disconnect power brake pushrod from top of brake pedal. Remove nuts attaching booster to firewall. Carefully lift booster unit away from firewall and out of engine compartment. To install booster, place into position and tighten retaining nuts. Connect pushrod to brake pedal and install brake light switch. Install master cylinder and vacuum hose. Test brake operation before normal use.