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300TDI Vacuum in cooling system.


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The cooling system in my 300TDI discovery seems to develop a vacuum instead of maintaining a completely full level.For instance,in the morning when the system is cold,I can remove the brass plug on the thermostat housing and hear the hiss of air as the vacuum is released.I normally just top it up with about a quarter or half a cup of water at the most.However after a few weeks of doing this,the level in the expansion tank is way overfull.The vehicle is not overheating and no apparent leaks any where.....expansion cap is new....so why does it not maintain its level from the expansion tank?

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Is the vacuum not just because the top of the thermostat housing is higher than the coolant level in the reservoir? As the brass plug is opened, air is allowed in, from the top, allowing the level to balance out with the reservoir? (or at least to the point, the air above the coolant, in the reservoir, is pressurised slightly, and stops the coolant dropping further in the thermostat housing)...

I'll caveat the above with a big "maybe"... :-)

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