Abstract: A seal for the drive shaft of a gear pump for melted polymeric materials is provided, which includes a reduced pressure zone, a mechanical packing seal, and a liquid filled chamber positioned in succession along the drive shaft. The liquid is a solvent for the polymeric material, and it is maintained under a pressure slightly above that in the reduced pressure zone. Thus a small quantity of the liquid tends to move axially into the reduced pressure zone where it dissolves any polymeric material leaking thereinto. A failure of the seal may be readily detected by monitoring the volume of the liquid, since any polymeric material leaking axially through the packing seal will be dissolved by the liquid in the chamber, thereby increasing the volume of the liquid.