GMC Sierra Denali Classic 2007 Owner's Manual page 312

If you will be pulling a trailer that, when loaded,
will weigh more than 5,000 lbs (2 270 kg) be sure
to use a properly mounted weight-distributing
hitch and sway control of the proper size.
This equipment is very important for proper vehicle
loading and good handling when driving. Always
use a sway control if the trailer will weigh more
than these limits. You can ask a hitch dealer about
sway controls.
Safety Chains
You should always attach safety chains between
your vehicle and your trailer. Cross the safety
chains under the tongue of the trailer to help
prevent the tongue from contacting the road if it
becomes separated from the hitch.
You may attach the safety chains to the attaching
point on the hitch platform. Always leave just
enough slack so you can turn with your rig.
Never allow safety chains to drag on the ground.
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Trailer Brakes
If your trailer weighs more than 2,000 lbs (900 kg)
loaded, then it needs its own brakes – and they
must be adequate. Be sure to read and follow the
instructions for the trailer brakes so you'll be
able to install, adjust and maintain them properly.
Your trailer brake system can tap into the
vehicle's hydraulic brake system only if:
•
The trailer parts can withstand 3,000 psi
(20 650 kPa) of pressure.
•
The trailer's brake system will use less than
0.02 cubic inch (0.3 cc) of fluid from your
vehicle's master cylinder. Otherwise,
both braking systems won't work well.
You could even lose your brakes.
If everything checks out this far, make the brake
tap at the port on the master cylinder that
sends the fluid to the rear brakes. But don't use
copper tubing for this. If you do, it will bend
and finally break off. Use steel brake tubing.