The performance of a radiator copper fin is considered as the braze joint between the fin and the brass tube is not perfect. The influence of different thermophysical properties of the brazing materials, created intermetallic compounds and possible air gaps is considered. Numerical methods for both two-dimensional and one-dimensional calculations have been developed. The finite volume technique is applied and in the two-dimensional case, boundary fitted coordinates are used. Heat conduction in the fin and braze joint coupled with convective heat transfer in a gas stream is analysed. Results in terms of fin temperature distributions and fin efficiencies are provided. It is found that the detrimental effect of a poor braze joint is not as large as reported previously in the literature.