A mandrel includes a mandrel body with a proximal major diameter barrel section, a diameter reducing/tapered portion and a distal minor diameter barrel section. The tapered portion connects the proximal major diameter barrel section and distal minor diameter barrel section. The proximal major diameter barrel section has a major diameter, the distal minor diameter barrel section has a minor diameter, the minor diameter less than the major diameter. The minor diameter is less than a diameter of an inner lumen of a coaxial catheter, and the major diameter exceeds the inner diameter of the outer lumen guidewire exit port of the coaxial catheter, and the tapered portion is sized to form a fluid seal the inner diameter of the outer lumen guidewire exit port. The distal minor diameter barrel section of the mandrel occupies and substantially seals and prevent flow through it to the catheter distal end guidewire entry port.