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If the expansion tank is draining or running dry after filling then..you must have a leak somewhere.a coolant pressure on the system would highlight that...However id say the easiest thing to check ideally here is the condition of the expansion tank cap itself,maybe replace it and see if that helps..its cheapest option..maybe it leaking and not holding pressure and allowing the coolant to evaporate...
Yes..well even if its not empty or is dropping then coolant pressure test would be advisable to rule out any leak..but please ensure the cap isnt at fault..like i say it could be faulty and not allowing the system to hold pressure..there is relief valve on the cap..so if faulty i can cause the coolant to evaporate when hot..
Thats because you need to cover all other logical areas first..you did say you had to fill the tank up did you not? plus you had no pressure..so whats the deal on that? is still occuring or what... Plus have you actually checked the feed and return pipe from the matrix to see if they are both hot or not? if ones hot and the others cold..then you got a blocked matrix..in which case you can try back flushing.