The Pumps as Turbines (PAT) is a well-established technology suitable for standalone micro-hydropower plants and energy recovery systems. But being lower performance than the dedicated turbines, there are continuous efforts to improve it keeping cost benefits intact. One of the recent modifications of the Back cavity filling plays a crucial role in the performance of PAT which is not investigated in detail. In the present paper, the PAT Back cavity is filled with a solid ring of various sizes and shapes (Back cavity filling) to explore its impact. The developed test facility is used to validate the experimental results with the numerical results for the base case. A numerical model after validation has been employed to investigate the impact of Back cavity filling on the internal flow dynamics and the PAT performance. Additionally, the study explored the influence of axial clearance on flow physics, associated losses, and the PAT performance, an aspect rarely discussed by researchers in the PAT mode. After Back cavity filling, secondary flow-based disk friction losses were reduced, leading to a 3.5 % increase in PAT efficiency. An analysis of the axial clearance showed that increasing it from 0.015 to 0.076 (mean axial gap/impeller radius) led to a 2.6 % reduction in PAT efficiency. This decline can be primarily attributed to elevated losses associated with disk friction, increased volumetric losses, and the formation of a mixing zone at the impeller inlet, which impeded the flow into the impeller's flow zone.