Pontiac 1999 Firebird Owner's Manual page 129

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Open the hatch and find the tab along the left side trim
panel. Lift up the tab and turn the valve counterclockwise
to release the hydraulic pressure. Turn the ignition key to
LOCK. You can now raise or lower your top manually.
Be sure to close the valve after installing your top so you
can raise or lower it electrically later.
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