When considering material models describing pressure, specific volume, temperature (pvT) behavior, that are used in engineering practice, one can still find that the used methods comprise sufficient potentials for improvement. The key point is, that material data is obtained at a quasi-equilibrium state, which does not reflect volume retardation at processing conditions. In scientific literature the KAHR-model is a well-established approach, in order to model time dependent specific volume of amorphous thermoplastics. The KAHR-model uses a generalized Maxwell retardation time spectrum along with empirically fitted shift factors for temperature and free volume. The determination of material-specific model parameters is costly, since time consuming temperature-jump experiments, that allow volume retardation at temperatures below glass transition at different pressures, would have to be performed. It is investigated whether the underlying rheological information could also be gained using a combination of dynamic mechanical analysis, temperature and pressure dependent viscosity measurements and pvt measurements. This work presents how the experimental data have to be obtained and discusses how they have to be combined in order to calculate a master retardation time spectrum with an appropriate shift principle.