[flatheadmurre]

Before ripping everything apart.
The antifreeze contains antifoaming has it been used up for any reason ?
So check it for exhaust/oil leftovers.
Sucking in air in a non pressurized system is possible adding a low pressure to the system will take that out of the equation.
Antifreeze has additives that is used up how fast depends on the enviroment its in.
If it was me i should have the coolant checked first and then start hunting down the fault source.