Sounds lie the tech was referring to combustion gasses leaking into the coolant.
All agree this should force the radiator cap to reach max pressure and vent out excess.
If the radiator is swelling before the cap releases, there must be something wrong with the radiator design or build.
Are you mounting the radiator solid to the chassis? If so, perhaps the frame is flexing placing extra stress on the radiator.
Here is a simple radiator test kit to check for combustion gas in the coolant.
https://www.amazon.com/Block-Tester-...YaAuw0EALw_wcB
