When the lid is closed, its sealing surface is firmly pressed against the edge of the expansion tank opening and if the safety valve is closed, there is no pressure loss.
If, when heated, the pressure in the cooling system exceeds 1.2 - 1.5 bar, the safety valve opens and some steam is released, and the pressure is equalized.
When cooling, the liquid again decreases in volume, and a negative pressure is created in the cooling system. To balance this negative pressure, there is a second valve in the cap. It opens at a negative pressure of 0.06-1.0 bar, so that outside air can enter the system.
Checking the safety valve
Those who do not want to bother with checking, in a suspicious case should simply replace the relatively inexpensive threaded cap. The workshop checks it with a pressure gauge:
Increase pressure.
At a pressure of 1.2-1.5 bar the valve should open.
It is only possible to roughly determine whether this valve is working.
With the threaded cap removed, squeeze the thick cooling system tube hard.
Put on and screw on the lid.
Release the hose.
If the compressed hose rounds back up, the valve is most likely OK.
If the coolant pipes appear flat and pinched in the morning before the first start, the vacuum valve is most likely faulty.
(The original publication in its entirety is posted on the website: «audimanual»)