[p_l]

From my understanding, most modern vacuum brakes involve two pressure loops with two cylinders on each brake. When connected and operating, one cylinder is responsible for brake force and the other acts as counter to it - in case of disconnect, the countering cylinder evacuates fast and results in braking.

However, if the system is not "primed" (for example after longer stay disconnected which gets both cylinders empty) the brakes are inoperational except for manual mechanical brake one is supposed to apply for any longer stay.