Chevrolet 2008 T Series Owner's Manual page 54

Diesel Engine Exhaust Brake
The exhaust brake is used to help slow the vehicle and
may assist the vehicle's foundation brakes. The use of
the exhaust brake will assist the braking of the vehicle
and prolong the life of the foundation brake lining
material.
Push the bottom of the button to turn the exhaust brake
on. When you push the top of the switch, the exhaust
brake will turn off.
2-16
The exhaust brake button
is located in the instrument
panel.
There is an indicator light on the instrument panel
similar to the symbol on the switch, that will come
on when the exhaust brake is active. See Exhaust Brake
Indicator Light on page 3-33 for more information.
When using the exhaust brake, the proper gear range
selection is important. The exhaust brake will be
most effective if the gear range selected is the lowest
possible range that does not allow the engine rpm
to exceed 2500 rpm.
If you have an automatic transmission and press this
button, the transmission will downshift to a factory
default setting of fourth gear. To change the default
setting, or for more information on factory default
settings, contact your dealer for assistance.
The accelerator and clutch pedals, on a manual
transmission, must be released in order for the exhaust
brake to engage.