Radiator

Was it overheating before you replaced these parts? Many engines need to have the trapped air pocket burped. Thermostats do not open in response to hot air. They respond to hot liquid. Look on or near the thermostat housing for a plug or sensor that can be temporarily unscrewed while you add coolant to the radiator.

Sounds like you may have addressed the symptom and not the cause. You can't get more than 15 psi in the system which is normal and desirable. There's a pressure relief valve in the radiator cap. If coolant was spraying from the radiator, that suggests it had a leak but that should not be due to excessive pressure.

The most common cause of excessive coolant flowing into the reservoir is a leaking head gasket. The easiest and fastest way to verify that is with a combustion gas test. Air is drawn from the radiator through a glass cylinder partially filled with a special dark blue liquid. If it turns bright yellow, combustion gases are leaking into the cooling system. One symptom is air bubbles appearing in the reservoir giving the appearance of boiling except that may occur when the engine is still cold. Also, there won't be steam associated with those bubbles.

That air that seeps in will also collect by the thermostat and prevent it from opening. That is what actually starts the overheating.