an inflatable penile prosthesis includes an implantable pump having a bypass valve . the bypass valve includes a cavity having a valve seat at a port , a poppet and a spring . the poppet includes a valve member and a stem extending from the valve member . in operation , the poppet includes a sealing position , where the valve member seals the port through contact with the valve seat , and a deflating position , where the valve member is displaced from the valve seat . the spring is configured to bias the valve member toward the valve seat , wherein the spring engages a portion of the stem while the poppet is in the deflating position .