Just checked it and the seal is in the bottom with the short end down and tall end up!! Was surprised how low the velocity was.
Best course of action might be to take the gun apart and replace all the seals.If it's an older 13xx the valve stem is brass with a soft insert that deteriorates.Replace it with a modern stem.Picked up these two 13xx guns recently both made in the late 70s.Received the 1377 this morning.Neither would pump up.Resealed them along with the valve stem.Also nipped the valve nose .020" and replaced the piston with the new type.It's slightly longer and added 25 FPS to both guns.1377 Classic......10 pumps 525 FPS 4.85 FPE1322 Medalist ...10 pumps 450 FPS 6.43 FPE