ok so yesterday replaced the coolant as well ,,,,,,,, it need around 6L of coolant including expansion tank and heater core,,,,,,,,, heater pipes was not fitted as the heater core got a leak around 1 year before ,,,,,,, that has been fixed during AC overhaul but pipes haven't been connected at that time ,,,,,,,, i use 3L of coolant mixed with 3L of water which i have been collected from a newly installed split AC unit ,,,,,,,,,, also remove the thermostat valve for flushing ,,,,,,, my plan was to flush the entire system with citric acid but haven't done that after seeing the clean condition of old coolant and pipes/radiator ,,,,,,, i think batter just flush the system with clean water and later blown out all the water trapped inside water jackets,,,,,,,, system good enough to go for next 5 years or so ,,,,,,,,