We present the results of a first principles based semi-classical study of electronic transport in Co/C/u/Co spin valves. The model is parameterized using Bloch states obtained from local spin-density electronic structure calculations and lifetimes appropriate to sputtered films. The approach is used to study the variation in giant magnetoresistance (GMR) with the individual layer thicknesses. Using values for the lifetimes for electrons in the Co and Cu layers that reproduce the resistivities of sputtered films, we are able to rationalize the variation in the current in the plane (CIP) GMR in a set of Co/Cu/Co spin valves produced under similar conditions. Important contributions to the GMR come from two sources, a channeling effect in the Cu which is a direct result of the different sized Fermi spheres for majority Cu and Co electrons, and from within a region in the Co layers within about one to two mean free paths of the interface. Unlike the CIP GMR, values for the current perpendicular to the plane (CPP) GMR are much larger and much less sensitive the variations in the individual layer thicknesses.