What is wrong when pressure builds up in the radiator?

Hello,

Is the radiator or coolant hoses pressurizing without the engine being hot?
If so, then most likely you have combustion gas leaking into the cooling system.
This is usually caused by a leaking cylinder head gasket.

Do you have enough coolant in the reservoir?

If the cooling system is building excessive pressure and the engine is overheating then we need to look at the radiator cooling fans, radiator/expansion bottle cap or thermostat.

To test for a possible blown cylinder head gasket a chemical block tester is used. See image below. These can be purchased on Amazon or vehicle spare part stores

How to use this equipment:
https://youtu.be/riF5hdhz0wM

https://www.2carpros.com/articles/how-an-electric-cooling-fan-works
and
https://www.2carpros.com/articles/how-an-engine-thermostat-works

Here is a good video on possible cooling system issues:
https://youtu.be/nhxrzN4eRC4

Hope this helps.

Cheers, Boris