GMC T-Series 1999 Owner's Manual page 157

CAUTION:
To help avoid injury to you or others:
D
Never let passengers ride in a vehicle that is
being towed.
D
Never tow faster than safe or posted speeds.
D
Never tow with damaged parts not
fully secured.
D
Never get under your vehicle after it has
been lifted.
D
Always use separate safety chains on each
side when towing a vehicle.
Here are specific towing instructions:
Towing Your Vehicle From the Front
(Front Wheels Off the Ground)
Before Towing
Block the rear wheels of the disabled vehicle. On
vehicles with air brakes, release the emergency brake
system by compressing the brake chamber springs as
outlined in this section. (Note: This is to prevent the
possibility of the emergency brake being applied
during towing.)
On vehicles with hydraulic brakes, release the parking
brake fully by moving the lever handle to the fully
released position.
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