A mock‐up has been constructed to test the functioning and performance of the Advanced Photon Source (APS) front ends. The mock‐up consists of all components of the APS insertion‐device beamline front end with a differential pumping system. Primary vacuum tests have been performed and compared with finite element vacuum calculations. Pressure distribution measurements using controlled leaks demonstrate a better than four decades of pressure difference between the two ends of the mock‐up. The measured pressure profiles are consistent with results of finite element analyses of the system. The safety‐control systems are also being tested. A closing time of ∼20 ms for the photon shutter and ∼7 ms for the fast closing valve have been obtained. Experiments on vacuum protection systems indicate that the front end is well protected in case of a vacuum breach. ©1995 American Institute of Physics.