1. Uncover a radiator and instead of it connect the device for check of tightness of system of cooling.
2. Create pressure of 103 kPas in cooling system.
3. Be convinced that pressure of 103 kPas in system of cooling remains long time.
4. Otherwise define a place of leak and eliminate it.
Prevention
The radiator cover for check of tightness of system of cooling should be released slowly, pressure in a radiator, and only after that to remove it did not decrease yet.