- The sealing surface of the screwed cap is firmly pressed against the edge of the expansion tank opening. The pressure cannot decrease.
- If the pressure in the cooling system exceeds 1.2–1.5 bar during heating, the high-pressure control valve opens. By equalizing the pressure, some water vapor can escape.
- When cooling, the liquid is compressed again, and a reduced pressure is created in the cooling system. To equalize the reduced pressure, there is a second valve in the cap. It opens at a reduced pressure of 0.06-0.1 bar, and atmospheric air can penetrate into the expansion tank.
Checking high and low pressure valves with improvised means
Those who do not want to spend effort on checking can simply replace the relatively inexpensive cap. In the workshop, it is checked with a pressure gauge:
1. Pump up the pressure.
2. At a pressure of 1.2–1.5 bar the valve should open.
3. With the cap removed, squeeze the thick fluid supply hose.
4. Put on and screw on the cap, release the hose.
5. Release the hose.
6. If the compressed hose rounds back up, the valve is OK.
7. If the coolant supply hoses are flattened in the morning before the first start, then the low pressure valve is probably not in order.