Abstract: The invention relates to leak isolating apparatus for multiple liquid cooled units in a coolant circulation system. The leak sensing portion of the apparatus comprises a supply flow meter and a return flow meter associated with each unit. The flow meters produce two variable electrical outputs proportional to the flow rates, and which are coupled to control logic for operating the isolating valves for the unit. The control logic compares the difference between the electrical outputs of the flow meters, and if the differences exceed a threshold, indicating a leak, operates the isolating valves. In the event of a sensed leak in a system having serially connected units, two serial valves are operated to prevent coolant flow to and from the leaking unit and a bypass valve is generated to allow coolant to flow around the leaking unit thus preserving the flow of coolant to the other units.