If your pump is not double sided (which the one in the picture looks like) the rubber valve can be reversed very easily by unscrewing the outer cap where you usually insert the valve and turning the little rubber valve around before screwing the outer cap back on.
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If your pump is not double sided (which the one in the picture looks like) the rubber valve can be reversed very easily by unscrewing the outer cap where you usually insert the valve and turning the little rubber valve around before screwing the outer cap back on.