Proper coolant level in the surge tank is half full, cold. If you try to keep the surge tank full, or close to it, the normal coolant expansion as system temperature rises (and hard hoses are normal) will blow off excess coolant.
Check the pressure cap for sealing ability - the rubber cap seals or the surge tank neck must offer a perfect surface for sealing - or the system never develops proper pressure leading to boilover at about 224 degrees.
Have you done a cylinder block exhaust gas test ? Rent the test kit from any large parts store, read the instructions, and bubble the test fluid for 20 - 30 seconds immediately after a drive. You sample the airspace over the coolant in the surge tank, not the coolant itself. ANY coolant enters the test fluid and you wash everything and start over.
If you want to see what a positive test for exhaust gas components looks like, sample the exhaust pipe gas at idle.
Check the pressure cap for sealing ability - the rubber cap seals or the surge tank neck must offer a perfect surface for sealing - or the system never develops proper pressure leading to boilover at about 224 degrees.
Have you done a cylinder block exhaust gas test ? Rent the test kit from any large parts store, read the instructions, and bubble the test fluid for 20 - 30 seconds immediately after a drive. You sample the airspace over the coolant in the surge tank, not the coolant itself. ANY coolant enters the test fluid and you wash everything and start over.
If you want to see what a positive test for exhaust gas components looks like, sample the exhaust pipe gas at idle.