A pump for metering a liquid is disclosed comprising a flexible tube connected to a container with a pressure relief valve located in the flexible tube. A housing defines an aperture for receiving the flexible tube with a compression surface located adjacent to the aperture. A reciprocating member has a sealing element and a pumping element. A drive moves the sealing element for pinching the flexible tube against the compression surface upstream from the pressure relief valve for trapping liquid between the sealing element and the pressure relief valve. The drive moves the pumping element forcollapsing the flexible tube against the compression surface for pumping the trapped liquid to discharge a metered quantity of the liquid from the pressure relief valve.