The minimum upstream pressure that is required to open a check valve is called the cracking pressure, and it can generally be anywhere between 1 and 5 psi. Check valves tend to be designed with this number in mind, and it can vary depending on your requirements. Flow rate determines the degree of opening on a check valve. Therefore, the higher the flow rate, the more open the check valve will become until it reaches it's fully-open (maximum) position.