Take the water bottle pipe out, after warming up your car. Take a fresh glass of water. When car is warm ( put AC on it ). Take that pipe into that glass of water and Observe carefully.
If it take out bubbles fastly or slowly, there is some leak in your cooling system.
I will recommend you something else first,
Park your car on some heavy slope. Start heater on at full temp, when engine is enough hot, take the radiator cap carefully with folded multi-layers of cloth. Then add water and let the burps of air come out of the radiator, when Burps stop then add water up the mark and close the cap.
Add water in coolant reservoir too but not more than water mark. If you add coolant instead of water that will be good.
Then take try. Sometime in old cars air trapped in the system, this air is really problematic in most case.
I believe its due to trapped air, Pls check it immediately and tell me if it still have trouble.