Drum filling with water when machine switched off
what you refer to as the overflow cap is the cover to the filter which you should remove regularly and check. however nothing to do with your problem.
the water could be coming from two sources:
first, if the drain is plumbed in under the sink, water can flow into the machine every time you empty the sink. the cure is to put the drain hose up as far as possible in a swan neck.
secondly, the intake water valves could be leaking and the only cure then is to replace them
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