Abstract: A valve test plug for connection with a pressure vessel. The plug includes a hollow valve body and a resilient valve core. The valve core includes first and second core portions bonded in end-to-end relationship and has a slit extending therethrough. The first core portion is exposed directly to the pressure and is softer than the second core portion which engages the valve body to close the slit or to seal on the probe when the probe is extending through the slit. The softer first core portion maintains a seal with the valve body and urges the second core portion into sealing engagement with the valve body end to close the slit.