Rad Caps??

The water pump doesn't really create pressure nor does it create a suction on its incommng side. Its main function is circulation by design. In certain conditions a water pump will spin while moving very little water. The thermostat has effect on minimum operating temp by blocking circulation through the radiater. Bypass hose allows the water to circulate within the engine while T-stat is closed. The system heat and pressure still reaches the radiater since there are no other gates. The rising temp creates system pressure. That pressure pushes water into the pump incomming side which is a good thing. More in equals more out.
The radiater cap is a safety feature since too much pressure would bust something. Lower system pressure only slows a leak. The lost pressure in turn lowers the volume of circulation.
Hope this helps.