I have a 04 A4 Quattro 1.8T that recently had the radiator and thermostat replaced as well as having the coolant system flushed 3 times due to an issue with the heater. After the system was bled the heater was working well for a couple of days, but then began blowing lukewarm air again. I have bled the system multiple times on a few different occasions now and the heater blows super hot air each time I bleed the system but then returns to lukewarm in a day or so. I am not losing coolant in the system and the engine is getting to proper running temp and staying in place. Are there any ideas as to why air keeps getting into the system and how can I fix it? Thanks for your help