Phil L pretended :
Forgot to add here

it doesn't require a fantastic amount of pressure, the machine simply opens a
valve for a set length of time, it can't 'know' how much water has passed
through in that time, unless it uses some sort of float valve, in which case
it doesn't matter how high or low the pressure is.

They don't open for a set time, the valve opens under the control of a
switch which detects the water level - basically a pipe leading to a
diaphramn, the pressure in the pipe as the water rises turns off the
valve and signals the controller to continue the wash process.

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Regards,
Harry (M1BYT) (L)
http://www.ukradioamateur.co.uk