did you not fit a bleed cap into your input and output hoses to the evaporator ? i incorporated 2 into my system when i fitted it. they are on the firewall at the highest point and can be bled at any time. when i do a full coolant change i remove the caps to release any trapped air. they were recommended to me by bernard lpg a former member on here. i bought mine on ebay but i have not got a link for you. you could do a search for hose bleed valves. i think these are maybe what you need. certainly i have had no problems with air locks since fitting mine ..
