GS 400 Brake Booster Repair or Replace

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My 1998 GS400 may require a new brake booster as a loud squeaking noise is becoming a regular occurrence when car is cold and brakes are applied. In warmer weather the noise disappears when the car is driven for a while. The brake booster and master cylinder are a single unit. Can anyone advise where such a new or rebuilt unit can be purchased. I understand that the units may be expensive and difficult to locate. I am located in Western New York and local distributors don't stock the item I am searching for. Any tech recommendations would be appreciated.