Algol-type binary systems are the product of rapid mass transfer between the initially more massive component to its companion. It is still unknown whether the process is conservative, or whether substantial mass is lost from the system. The history of a system prior to mass exchange is imprinted in the photospheric chemical composition, in particular in the carbon to-nitrogen (C/N) ratio, We use this to trace the efficiency of mass-transfer processes in the components of a classical Algol-type system, delta Librac. The present analysis is based on new spectroscopic data (ground-based high-resolution echelle spectra) and extracted archival photometric observations (space-based measurements from the STEREO satellites). In the orbital solution, non-Keplerian effects on the radial-velocity variations were taken into account. This reduces the primary's mass by 1.1 M circle dot(w) (similar to 23 per cent) significantly in comparison to previous studies, and removes a long-standing discrepancy between the radius and effective temperature. A spectral disentangling technique is applied to the echelle observations and the spectra of the individual components are separated. Atmospheric and abundance analyses are performed for the mass-gaining component and we found C/N=1.55 +/- 0.40 for this star. An extensive set of evolutionary models (3.5 x 10(6)) for both components are calculated from which the best-fitting model is derived. It is found that the parameter that quantifies the efficiency of mass-loss from a binary system, is close to zero. This means that the mass transfer in delta Lib is mostly conservative with little mass-loss from the system.