Hydraulic-pneumatic hybrid powertrains provide an opportunity for combined high power and high energy regenerative braking systems for heavy duty vehicles that need to transpose both highway and urban areas. The challenge in designing these systems resides in the proper sizing, integration, and control of the components to exploit with high efficiency the different energy and power range available in these applications. In the novel concept proposed here, braking energy is recovered by a hydraulic system and stored in a hydraulic accumulator and in an air reservoir. While the hydraulic system shares the vehicle propulsion in parallel to the internal combustion engine, the compressed air is used to power auxiliaries in power-assist mode. The conception, modeling, sizing, and system integration are presented, relying on commercially available components. The lumped parameter simulation model written in MATLAB/Simulink is first validated by comparing measurements and predictions for a laboratory system. Then, two conditions of braking energy recovery for a 19 tonnes bus are analyzed: a full stop, typical of urban driving, and a downward road slope, typical of highway driving. Results indicate that the system proposed is able to store 69% of the available energy during full stop and 14% in the highway downward slope. However, although the storage efficiency is smaller in the downward slope, the total energy recovered is 2.8 times larger than the energy recovered after full stop. Also, while the pneumatic energy stored is only 20% of the energy stored in the hydraulic accumulators after full stop, it is more than twice for the road slope. These results indicate an opportunity for significantly improving the overall energy efficiency of delivery trucks and buses.