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According to the WSM you pull the locking clip and push the locking pin out. Both my WSM's brake booster removal pages are dated 86. Page before and after are dated 90. I could be missing an update. I even checked the online versions. I guess I could dig up Jim Morrison's CDs and see if there was an addendum I am missing.
On the 90 there is a support plate not found on earlier models that prevents the pin from sliding out all the way out. The pin itself will move a half an inch or so before the it hits the vertical support. Sorry about the picture quality.
I know that I've done this, but don't remember the 'trick'- if you loosen the whole booster from the firewall, can you maneuver it laterally far enough to get the clevis off the end of the pin?