A piston member for a high-pressure pump for pumping fluid in a sample separation apparatus, wherein the piston member comprises a piston configured for being mountable to reciprocate in a piston chamber for displacing fluid, and a sealing for sealing between the piston member and the piston chamber when the piston member is mounted in the piston chamber to reciprocate, wherein the sealing is mounted on the piston so as to reciprocate together with the piston.