Methods and devices to facilitate positioning of a catheter into a vessel. Devices include axially concentric assemblies of a piercing needle and a dilator that guides an outer catheter into a vessel, such as a blood vessel. The assemblies described herein can include retraction mechanisms and/or lock mechanisms to control needle positioning during catheterization processes as well as improved valves that prevent leakage of fluid from the proximal end of the devices. The devices and method include the use of novel valves to prevent undesired leakage of fluids through the proximal end of the catheter.
