The water pump does not develop any pressure. Its only purpose is to push the coolant around the system. The pressure comes from the expansion of the coolant as it heats up. It goes into the reservoir so the level there will go up, then, when the coolant cools down, it contracts and is drawn back into the engine, and the level in the reservoir goes back down.
The fact you are having to add coolant with no visible leaks suggests a cylinder head gasket is leaking. Your mechanic can perform a quick chemical test to verify that. The test involves drawing air from the radiator, (reservoir on GM cars), through a glass cylinder with two chambers partially-filled with a special dark blue liquid. If combustion gases are leaking into the cooling system, the liquid will turn bright yellow.
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Sunday, May 8th, 2016 AT 9:13 PM