1999 GMC Sonoma Owner's Manual - Page 260

yellowblue When to Check and What to Use How to Check The proper fluid should be added if the level does not reach the bottom of the diaphragm when it's in place in the reservoir. See the instructions on the reservoir cap. 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. Refer to the Maintenance Schedule, Owner Checks and Services, to determine how often you should check the fluid level in your clutch master cylinder reservoir and for the proper fluid. See "Owner Checks and Services" and "Recommended Fluids and Lubricants" in the Index. 6-22