[benz660r]

your losing fluid somewhere and causing air to get into the system..... if your not leaking fluid and all hoses are in good condition not crushed or bent or anything of that sort, then your loosing coolant in your heads. Best solution is to replace the gaskets with the newer ones as stated above by ryansquick6. If you disagree, then i leave you with the question of what else is left in this system to fail? We know the rad, therm, and waterpump are all functioning, hoses are good, system is purge at the start.....