Abstract: A bleed back control assembly and method for controlling blood loss during catheterization procedures includes a side arm body connected at the proximal end to a seal body. The side arm body comprises one or more branches, each having a lumen, and a finger rest on the exterior of at least one branch. The seal body comprises a cap assembly and a seal assembly. The seal assembly comprises a clamp seal and a bleed back seal, both made of elastomer and held by a seal holder. The cap assembly comprises a threaded cap and a funnel cap. The threaded cap is connected to the seal holder. Rotation of the threaded cap causes the clamp seal to open or close. The funnel cap comprises a dilator, and pressing the funnel cap causes the dilator to open an aperture in the bleed back control seal. A spring, wound around the dilator, returns the funnel cap to its original position, thus closing the bleed back seal. The bleed back seal self-sizes to devices introduced through its aperture.