Using the same pump, you can check the opening pressure of the valve in the cap. Place the pump on the cover and operate it. At a pressure value of 0.9 bar, the pressure should drop immediately, i.e. the valve opens in the lid. If it doesn't, the cap needs to be replaced. A valve that does not open quickly leads to engine overheating. If the valve is stuck open, the system cannot be pressurized.
Checking the radiator cap with a pump.