Why does water build up in water reservoir?

Car is ford focus sedan 2003 1.6engine
So we changed the water reservoir recently and now water builds up in the reservoir.

Replace the radiator cap. If it does not have one, make sure you service the coolant to the correct level when the engine is cold.

Supposed to

The reservoir tank is a surge tank. When the radiator gets hot it will send a little water to the surge tank. When the radiator cools off it will suck some back into the radiator. It sounds like yours is working properly. There's a level mark on the surge tank which is wear you should fill to.

Coolant was serviced too. Water goes in reservoir and does not go out? The cap is sealed tight and water fills the reservoir completely after short drive. It does not recede so I had to drain manually.

Since this occurred after changing reservoir it likely we have a pressure leak in system. E.g. Hose not tight> cap not sealing? A new pressure cap may be worth investing in.
if it was occurring before we changed reservoir? Then its likely we have a head gasket issue.

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