OK....silly question, but does water come out of the drains when you are hooked up to "city water?" If not, your valves are either not working or plugged or they are not the low point drains. Plugged is unlikely with piping the size of the water lines. Are you sure you're opening the low point drains? I don't mean to offend, but I've never heard of both hot and cold water drains being plugged. When you have air on the lines, do you hear any kind of air leaks? Do you get full air pressure out of all of the water fixtures, including the toilet? Weird...
Let us know what you find....