I have read the notes that say “It is essential you do not overfill the expansion tank. It should be between a quarter and a third full when cold. If it is too full then you can get syphonage from the radiator and then consequent overheating.”
So, I always check the expansion tank is between a quarter and a third full. And I top up the radiator at the brass plug. It has a new seal, the expansion tank pressure cap is new and all the hoses are new. I use antifreeze mixed with de-ionised water. The car has twin highly efficient electric fans fitted which cut in if the engine hits slightly above normal temperature.
But, here’s the problem. After a run, the expansion tank is always full and starts syphoning cooling water into the ground. How can I prevent this happening please?
The car is a 1976 factory original V8 with a standard 3.5L engine.