I have a 2002 Chev S-10 with heater problems. Have replaced thermostat twice, changed radiator cap, flushed radiator, heater core and lines. Still will not heat. After worked on everytime heater will work but 1-3 days later will not blow hot air. Have had it in 3 different shops. Some have said air in lines or core, need to purge. But how do you PURGE? I need help is getting expensive and still no heat, Got any ideas??? Thanks