Hello John, I did some work on my cooling system when rebuilding my M type. The pressure relief valve on top of the radiator was not very good and the system would not hold any pressure. This was adapted to run a hose to a header tank fitted with a 7lb radiator cap. The overflow from the header tank ( water expansion ) goes to a catch tank. My system holds about 41/2 gallons about 22 Ltrs, when full to the brim including the header tank. When up to temperature about 185/190F on the gauge and the 7lb cap releasing excess pressure ( water expansion ) It displaced approximately just over 1Ltr . When the system was left to go stone cold again, the water in the overflow tank returned to the cooling system via the 7Lb radiator cap ( water contraction ). I was using a 50/50 mix of antifreeze / water. If your cooling system does not have any form of catch tank or a properly working radiator cap any water that is expelled is lost to the ground. When you check the radiator again you will see that the level has dropped to just above the core and could be misleading ( no other leaks accepted ) do not trust the original relief valve. Hope this helps you out kind regards,
Charles
