If you have done everything including the thermostat cap and squeezed the bubbles out, then it might be a head gasket. Just go to a radiator shop, I trust Sims Radiator shop in Lawrenceville, and get a block check done. They put a dye into this container and squeeze the air out of the cooling system onto that dye. If the dye turns a certain color, then your head gasket is blown and the exhaust gases are mixing with your coolant.