As I understand, the radiator cap has a spring so it can hold pressure. When the car heat up and the pressure exceed the pressure the spring can hold, the coolant will flow to the reservoir. When the car cools down, the fluid contract and create a vacuum. Then coolant can be sucked back through a vacuum valve in the cap. This is what I understand.
Here is a thing that confuses me. When the car is cold, I can squeeze the upper or low radiator hose. When I squeeze it, I can see coolant goes in and out of the reservoir. This is pretty confusing to me. The radiator cap should withstand 15psi pressure. So if I light squeeze the hose, The fluid should not flow out to the reservoir because a light squeeze should not create too much pressure. Since no fluid is out, when I release my hand from the hose, there should not be any vacuum and fluid should not come back. So when the car is cold, I should not be able to see the fluid go in and out of reservoir if I squeeze the radiator hose.
So why did I see this happening? Is my understanding of how radiator cap work wrong or is it because my radiator cap is faulty?