GMC 2003 GMT560 T-Series Owner's Manual page 247

Check air brake relay valve operation and check for
leaks (or every month, or every 300 hours, whichever
occurs first).
Clean air brake application valve (or every 3 months,
or every 300 hours, whichever occurs first). Lubricate
linkage.
Air brake chamber service (or every 2 months,
whichever occurs first). (41)
Trailer brake hand control valve service (or every
3 months, or every 900 hours, whichever occurs
first). (42)
Rear axle air shift motor service. (11)
Air intake system service (or every 24 months,
whichever occurs first). (4) (5) (24)
70,000 Miles (112 000 km)
Change engine oil and filter (or every 12 months,
whichever occurs first). (2) (3)
75,000 Miles (120 000 km)
Inspect engine air cleaner filter change indicator. If
necessary, replace the filter. If vehicle is driven in
dusty/dirty conditions, inspect filter change indicator
at every engine oil change. See Engine Air
Cleaner/Filter on page 5-12 for more information. An
Emission Control Service. (See footnote †.)
Chassis lubrication service (or every 6 months,
whichever occurs first). (12)
Check fluid levels (or every 3 months, whichever
occurs first). (1) (6)
Replace fuel filter. (3)
Steering system service. (13)
Front and rear suspension service. (14)
Spring-to-axle U-bolts and shackle bolts service. (16)
Exhaust system service (or every 6 months,
whichever occurs first). (3) (4) (17)
Wheels and tires service. (15)
Hydraulic brake service (or every 6 months,
whichever occurs first). (7) (46)
Parking brake service (or every 6 months, whichever
occurs first). (8)
Air brake service (or every 6 months, whichever
occurs first). (39)
Air brake automatic slack adjuster service (or every
500 hours, whichever occurs first). (40)
Check air brake relay valve operation and check for
leaks (or every month, or every 300 hours, whichever
occurs first).
Clean air brake application valve (or every 3 months,
or every 300 hours, whichever occurs first). Lubricate
linkage.
Air brake chamber service (or every 2 months,
whichever occurs first). (41)
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