In a control system for operating a pump, the pump is pneumatically driven to pump hydraulic fluid to pressurize a closed vessel. The control system includes inlet and outlet ports, a pneumatic regulator, a pressure transducer, and control circuitry. The inlet and outlet ports are configured to receive inlet and output outlet pressurized air, respectively. The outlet pressurized air powers the pump. The pneumatic regulator is disposed between the ports, and configured to receive the inlet air and output the outlet air. The pneumatic regulator includes a valve that modulates pneumatic pressure based on a control signal. The pressure transducer provides a hydraulic pressure signal indicative of pressure at the output of the pump. The control circuitry is configured to adjust a parameter of the control signal based on this hydraulic pressure signal, and output the control signal to the pneumatic regulator to indirectly control operation of the pump.