How old is the car? If its a very old car, then after all flushing with citric acid and neutralizing, leak test the cooling system under pressure. BBecause Micro leaks don't show up unless the car reaches optimal running temperature.
I would advice you to dismantle the cooling system and inspect the intake manifold for any leaks and the tikli or small plate like thing under it for leaks and inlet and outlet of the intake manifold and also replace all rubber hoses, metal pipes and water bodies of the cooling system
Replacing is recommended and close inspection is recommended of all cooling system parts because if the car is old and has has been running on water instead of coolant then it must have 35+ punctures
Also let me warn you before hand that change the thermo switch (temperature needle switch) because it will have accumulated gunk and rust all these years on it and it will not show correct reading on the guage and incase you are going on a long drive and all water leaks out slowly and the remaining becomes steam then in case of an over heat and while the engine will be boiling oil and fuming vapors out from the gasket area, the needle will only rise up till 75% mark...so you will never be alarmed till you feel jerks and the car comes to a halt ... and the gasket is gone..