1997 GMC Sonoma Owner's Manual - Page 260

When to Check and Whatto Use Rear Axle When to Check and Change Lubricant Refer to the Maintenance Schedule to determine how often to check the lubricant and when to change it. See "Scheduled Maintenance Services" in the Index. How to Check Lubricant Refer to the hintenance Schedule, Owner Checks and Services, to determine how often you should check the fluid levelin your clutch master cylinder reservoir and for the proper fluid. See "Owner Checks and Services" and "Recommended Fluids and Lubricants" in the Index. How to Check The proper fluid should be added if the level does not reach the bottom of the diaphragm when it's inplace in the reservoir. See the instructions on the reservoir cap. If the level is below the bottom of the filler plug hole, you'll need to add some lubricant. Add enough lubricant to raise the level to the bottom of the filler plug hole. 6-22