GMC T-Series 1999 Owner's Manual page 197

5. Using the handle located at the bottom of the cab at
the back, rotate the cab forward.
A. Safety Lock Pin
B. Cab Support
CAUTION:
A cab not properly locked in the tilted position
could fall, causing serious injury. To insure that
the cab is securely locked in position and won't
move, the safety lock pin (A) must always be
inserted into the cab support (B) when the cab is
in the tilted position.
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6. Tilt the cab all the way
forward until the cab
support under the cab
locks fully.
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