A connection module (10) for installation in a closed breathing circuit of a closed-circuit breathing apparatus (1000), includes an inlet port (12), for connection to a first closed circuit element, an outlet port (14), for connection to a second closed circuit element, a maintenance fluid inlet (16), and a maintenance fluid outlet (18). A maintenance device may be provided with the connection module and a controller (30) and control module regulated valves (40).Maintenance fluid is circulated through the closed breathing circuit by the controller and valves in a state in which the connection module (10) is installed in the closed-circuit breathing apparatus. A method for maintaining a closed-circuit breathing apparatus includes opening the closed-circuit breathing apparatus at a point, installing the connection module or the maintenance device, and introducing maintenance fluid into the closed breathing circuit of the closed-circuit breathing apparatus through the connection module or the maintenance device.