Pneumatic actuation has gained popularity in robotics, for its low maintenance, high efficiency and compactness. A recent direction is to build pneumatically driven non-magnetic platforms for robotic rehabilitation monitored by magnetic resonance imaging (MRI). Control of such systems is challenging as they involve long transmission lines to satisfy MR-compatibility requirements and direct pressure measurement at the actuator is not available. This paper presents a control algorithm that provides an accurate impedance control for pneumatic systems with long transmission lines. The algorithm involves a pressure observer based on force and displacement feedback to compensate for the lack of pressure measurement. The observed pressures are utilized in a robust controller for force tracking. The described method is tested on a system with 7 meters long transmission lines. Impedance control using the presented observer achieved stable force tracking up to 1.5 Hz, lowering mean squared error by up to %25.