1994 GMC Suburban Owner's Manual - Page 224

How to Add Coolant to the Coolant Surge Tank If you haven't found a problem yet, but the coolant level isn't at the COLD mark add a 50/50 mixture of clean water (preferably distilled) and a proper antifreeze at the coolant surge tank, but be sure the cooling system, including the coolant surge tank pressure cap, is cool before you do it. (See "Engine Coolant" in the Index for more information about the proper coolant mix.) L 5-20