Proportional delay differentiation (PDD) aims to maintain pre-specified packet queueing-delay ratios between different classes of traffic at each hop. Existing rate-allocation approaches for PDD services assume the average queueing delay of a class is inversely proportional to its service rate. This assumption may not be valid in a non-heavily loaded system. In this paper, we present a rule-basedapproach to provide consistent PDD services under various load conditions. In this approach, the service rate of a class is adjusted according to a set of control rules. Simulation results demonstrate that, in comparison with other rate-allocation approaches, the rule-based approach is able to provide PDD services under light, moderate, and heavy load conditions consistently.