Had a similar problem with a Buick 215 V8 engine in the rear of a VW bug with the radiator in front in the spare tire area with air ducts to the radiator and long runs of coolant pipes to the rear engine. Hard to get and KEEP the air out of the system. Ended up puting bleed ports in different points and also had to open these ports about every 3/4 months to bleed air that would get trapped. Just a change the oil additional job.With a complex cooling system, there needs to be bleed ports at certain points to make sure there is no air in the system. Open the bleeds, put coolant in and once the first bleed starts purging, close it off. Move on to the next until the top. There has to be a bleed port as the highest point of the coming system. A car is usually the radiator cap, but I have cars that it’s on the engine.