Radiator Connections

Note, go back and look at your question and then think again.

is the reason of positive suction for water pump correct ?

The 1400cc engine fitted to the Volkswagen Polo does not have a top-up connection. If the fluid level requires topping-up, then it goes in via the radiator cap when the engine is cold enough to open it without risk of injury.

The coolant is a hazardous substance. The make up tank is a collection point for any that blows by the pressure cap on the radiator. In the past we just dumped it on the ground. Hose ran off it down the side of the radiator and expelled any fluid that made past the cap into our environment. The tank also provides a inspection point to see if the radiator is full with out the hazard of opening it up with a hot engine as it's under pressure. This is not a sure inspection but the idea is as the fluid expands and contracts in the radiator it will expel and draw from the tank. So if there is a good fluid level in the tank you can be reasonably insured that the radiator has fluid.

The pump suction has nothing to do with it. The fluid level is above the suction of the pump so that provides positive input.

Smaller engines also have different maintenance to be preformed on them as stated by the OEM. More frequent checks of fluid levels as stated in the daily operation check list. The radiators are smaller so there is less fluid to expand. So there not as much need for the tank.