1 Some cooling systems have a nonpressurised expansion bottle, which draws coolant into the system when required and also stores any excess coolant when there’s too much. This bottle can get blocked with dirt, so it’s worthwhile removing and cleaning inside it.
2 Detach any pipework at the expansion bottle, then look for how it’s mounted in position. There will probably be one or two nuts or bolts to undo before it can be lifted out of the engine bay.
3 With the expansion bottle removed, fill it full of clean water, then try to clean inside with a bottle brush. Any dirt inside could block the pipe that’s connected to the coolant system.