The plastic boot on the brake booster can harden over time, but as long as you don't touch it, it can last a very long time. It's only during disassembly that you have to face up to the fragility of the material, and that pieces may come loose. The door is then open to moisture and all kinds of dirt that the brake booster doesn't like.
If this is the case, or if the sleeve/boot shows cracks due to dryness, there's no need to buy a complete servomotor; you can replace the aerator fitting, including the sleeve, separately.
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