Q&A

I have seen the plastic housings and expansion tank leak. The way an automotive shop would locate the leak is by removing the radiator cap and attaching an adapter that screws on in place of the radiator cap. Something that looks like a bicycle pump with a pressure gauge attached to it is used to "pump up" pressure to simulate pressure generated when the cooling system is at operating temperature, the leak is then found sometimes it is obviously leaking from a coolant hose or cracked housing, failed gasket or water pump, sometimes the engines cooling system needs to be inspected from below or inspected using a mirror and flash light.

Most modern methods of coolant "flushing" are non intrusive and don't require cutting hoses as was done in the past, but it would depend on the equipment, the shop, and the person performing the service. Just the removing of sediment and maybe some sealers (while flushing) that were in the cooling system can "reveal" leaks. Some manufacturers that have factory installed coolant sealer recommend that a sealer be put back in the cooling system after a flush to prevent leaks. Bottom line is it depends on where the leak is as to what may have caused it.