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http://bmwe32.masscom.net/maxf_websi..._breakdown.htm
How to get it apart
To get your booster in this state, you've got to use vice-grips to loosen that little grub-screw on the bottom of the body - it holds the endcap K in against the pressure of return spring H.
BE CAREFUL - that's a powerful spring, so use a rag over the end to stop bits flying and don't put your face close!!