[Barrie]

13lb pressure cap on expansion tank and plain cap on radiator.

The plain cap is to refill the radiator only.
The pressure cap is to allow excess pressure in the system to vent.

Expansion of the water from the radiator sends it into the pressure tank and returns when it contracts.
If pressure gets above 13lbs sq inch above bar, first air, then water vent through pressure cap on the expansion tank.
If you put the pressure cap on the radiator then the water in the radiator will only flow to the expansion tank when the pressure is above 13lbs sq inch and, crucially, it cannot return as the pressure cap is a one-way valve and so any water which makes its way to the expansion tank cannot return thus negating its reason for being.

Pressure caps on radiators are only for systems with no expansion tanks.

PS there are at least two different depths of pressure caps. If yours is too shallow it will not allow pressure to build up and so overflow more easily. Identification of the correct one is by observation and feel: when you insert the cap it should protrude above the rim of the aperture and you then press down to compress it and then turn it to close. Also note that the springs in the cap weaken over time so should be replaced regularly.