At Least Once A Year - GMC 2005 T-Series Owner's Manual

Underbody Inspection
Corrosive materials used for ice, snow removal and dust
control can collect on the underbody. If these materials
are not removed, accelerated corrosion (rust) can occur
on underbody parts such as fuel lines, frame, floor pan
and exhaust system. At least every spring, flush these
materials from the underbody with plain water. Take care
to clean well any areas where mud and other debris can
collect. Sediment packed in closed areas of the frame
should be loosened before being flushed.
Engine Cover Check
Check that the cab's engine cover and seal (if equipped)
are not torn or damaged. Be sure that the cover is
clamped down firmly to the floor.

At Least Once a Year

Key Lock Cylinders Service
Lubricate the key lock cylinders with the lubricant
specified in Part C.
Underbody Flushing Service
At least every spring, use plain water to flush any
corrosive materials from the underbody. Take care to
clean thoroughly any areas where mud and other debris
can collect.
Tractor Protection (Breakaway) Valve
Check
On air brake models, remove, disassemble, clean and
inspect the tractor protection (breakaway) valve.
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