A method of preventing siphoning of a liquid through a pump 23 having an inlet and an outlet, the outlet being coupled to a discharge line 24 having an air inlet valve 10 that admits air to prevent a body of water to build up and cause the liquid to siphon from the discharge line back through the pump. The liquid may be condensate collected from a condenser 20 of an air conditioning installation, of particular benefit for use with piston pumps, which may be used in coffee machines. The admission of air through the valve does not allow a body of liquid build up downstream of the valve such that there is an inadequate weight of liquid to cause siphoning from the discharge line upstream of the pump. Therefore, the pump always contains liquid and prevents it from running in the dry state. Also claimed is an air inlet valve (fig 4) which may be a non-return or one way duck bill valve (33) made of an elastomeric material.