1999 GMC Sonoma Owner's Manual - Page 221

yellowblue 3. Fill the radiator with the proper DEX-COOLR coolant mixture, up to the base of the filler neck. (See "Engine Coolant" in the Index for more information about the proper coolant mixture.) 4. Then fill the coolant recovery tank to the ADD mark. 5. Put the cap back on the coolant recovery tank, but leave the radiator pressure cap off. 5-17