A device (1) for testing pneumatically operated brakes of a first vehicle (22) and of a second vehicle (23) being connected and having interconnected brake systems. A device supply line (3b) and a device control line (4b) are arranged for connection to respective corresponding supply and control lines (3a, 3c, 4a, 4c) in the vehicles such that the device (1) is fluidly connected into and between the brake system of the first vehicle (22) and the brake system of the second vehicle (23). A method of activating the brakes (5) of a trailer unit (23) which is connected to a tractor unit (22), without activating the brakes of the tractor unit (22), provides braking action by employing the device (1), and includes remotely activating the first valve means (9) in order to provide braking action.