That is how the CSV works. It makes the pump think it is in a deeper well, so it can only pump the amount of water needed. Your problem is the pressure relief valve should be downstream of the CSV, not upstream. Upstream of the CSV the backpressure will be whatever your pump can build, usually about 100-150 PSI. Downstream of the CSV the lines and the pressure relief valve will only see 40 to 60 PSI, and a 75 PSI pressure relief valve won't pop off.