Heating up

Just dealing with this on another brand. Custom rad to fit truck. Boiling over. Cap was not holding pressure.
Tested cap. Was good. Cap wont seal in rad. I repaired bung. Was a shitty job done building rad.
Get an air regulator,dial it down to 10 psi and pressurize rad through overflow tube.
It should hold it.
As a test, i slipped an o-ring between rad cap seal and the metal disc that pushes on seal.
Like making the seal thicker. It held pressure.