Single-phase flow numerical models of rapidly filling combined sewer overflow storage tunnels can predict trapping of large volumes of air; this occurrence is generally associated with the reflection of a hydraulic bore at a tunnel transition. Because the models do not actually consider the presence of air in the predicted void, water hammer pressure transients are subsequently predicted following the closure of the void and the predicted pressure variations can indicate both significant high and low pressures. The actual presence of air within the void will result in air compression, rendering the simulation results invalid. However, it is known from previous studies that compression of air pockets can also result in large system pressures. A laboratory study was performed to examine the pressure variations associated with the impingement of a pipe-filling hydraulic bore on a discrete air volume of varying size. It was found necessary to devise a particular laboratory setup to create such a hydraulic bore. Both maximum and minimum air pressures were obtained from the pressure records and these were found to be a function of the trapped air volume. A dimensional analysis was developed to provide a basis for interpretation of the experimental data. Although this analysis was based on some explicit assumptions, the experimental results are reasonably well collapsed into a single dimensionless relation. Since the dimensional analysis is based on conditions at the hydraulic bore front at the instant of air entrapment, a method for utilizing the results of single phase numerical model predictions to estimate pressure rises due to air compression is indicated. This application suggests that both maximum and minimum system pressures can be a concern in the design of storage tunnels. Limitations of the experiments are discussed.