In this paper a refinement to the delay-time-based PM model is presented to account for the downtime incurred at failures when estimating the expected number of failures over a PM inspection period. Extensions to PM models have been made in the context of downtime modelling. Previously, in downtime modelling using delay time, it has been generally assumed that the downtime of failures is small compared with a PM cycle length and, therefore, assuming that defects continue to arise uninterrupted during a downtime period has negligible consequences. However, in some cases, it is possible that the cumulative downtime of failure repairs is not sufficiently small. The paper presents revised models for perfect and non-perfect homogeneous processes, and for a perfect non-homogeneous process. Numerical examples are provided to highlight the differences. The revised downtime model is a more sensitive model with which to determine the actual downtime or cost.