With preparation of Hi-Lumi LHC fully underway, and the FCC machines under study, accelerators will reach unprecedented energies and along with it very large amount of synchrotron radiation (SR). This will desorb photoelectrons and molecules from accelerator walls, which contribute to electron cloud buildup and increase the residual pressure - both effects reducing the beam lifetime. In current accelerators these two effects are among the principal limiting factors, therefore precise calculation of synchrotron radiation and pressure properties are very important, desirably in the early design phase. This PhD project shows the modernization and a major upgrade of two codes, Molflow and Synrad, originally written by R. Kersevan in the 1990s, which are based on the test-particle Monte Carlo method and allow ultra-high vacuum and synchrotron radiation calculations. The new versions contain new physics, and are built as an all-in-one package - available to the public. Existing vacuum calculation methods are overviewed, then the steady-state and time-dependent algorithms behind the ultra-high vacuum simulator Molflow are presented. Some practices to tackle the most common problems that arise when simulating large systems are also discussed. Results are compared to theory, and validated through two experiments. Next the the main steps of synchrotron radiation simulations are presented. Properties of SR are summarized, along with optimizations that allow simulating the rather complex underlying physics at a higher speed. The resulting software's photon generation algorithm is benchmarked against published data. The phenomenon of photon stimulated desorption and its literature is overviewed, then two dedicated photodesorption experiments carried out in KEK (Tsukuba, Japan) are presented: one with six room-temperature samples and an other at liquid nitrogen temperature. A simple synchrotron radiation calculation is performed for the LHeC interaction region, allowing to compare Synrad+ results with published analytic calculations. Then the calculations are repeated for a more precise geometry description. The pressure profile of a crotch absorber of the recently started Max IV light source is calculated using Molflow+ and Synrad+ together. Finally the pressure analysis of the SuperKEKB interaction region is presented, consisting of modeling the vacuum chamber and the optics, calculating synchrotron radiation, then performing vacuum simulations. It is confirmed that pressure is expected to meet the design requirements during operation of the machine.