Engineered multilevel systems (MLS) are one of the few viable options for collecting spatial and temporal datasets in 3-D complex groundwater flow systems. However, these monitoring systems present challenges for hydraulic testing due to small port tubing diameters. In this study, equipment was developed to enable pneumatic slug testing of a G360 MLS, fitted with 1/2-inch ID open tubes that extend from the monitoring ports to the surface. Of the eight ports tested, four exhibited overdamped slug test responses, while underdamped responses were observed in the remaining four ports. It is expected that friction in the small diameter tubing is not negligible, and additional flow constrictions are introduced when installing a transducer in the tubing, effectively changing the open tube geometry to an annulus around the transducer body and around the transducer cable. Steady flow models for annular flow, widely available in the literature, were used to assess tube friction for the overdamped tests. However, for underdamped tests that oscillate quickly, a mathematical solution for unsteady oscillatory flow through an annulus was derived. The results of this study show that it is important to account for frictional and inertial losses to obtain good transmissivity (T) estimates from slug tests conducted in small diameter tubing. Assuming steady flow through small annuli will not introduce appreciable error when calculating the water level; however, if steady flow through the open tube below the transducer is assumed when calculating the formation head, T values can be underestimated as much as an order of magnitude.