I don't have a res', I just have a fill tube inplace before the pump inlet that acts as an airtrap (1/2 PVC tube connected to loop with a 15mm copper T ), people say the best place for one is at the highest point, mines at the bottom and works fine

. It does'nt have to be too long either (4~6inch is fine), just top it up with water during the first 5mins of running...