Volvo VNL 780 Operator's Manual page 328

318 OPERATION
The parking brake spring tension is sufficient to hold a fully loaded vehicle on maximum
grades permitted on modern highways, provided the brake drums and linings are in good
condition and the brakes are properly adjusted.
CAUTION
DO NOT use the spring parking brakes
if the service brakes are hot, such as
after descending a steep grade. Also, do
not use the spring parking brakes during
freezing temperatures if the wheel brakes
are wet. To do so could damage the
brakes, if hot, or may cause them to freeze
during cold weather.
W5001430