I have this pump for circulating the water in a fountain. The pond/fountain designer told me to get the pressure up to 20 psi and to keep running/shutting down/bleeding from a faucet above the pump until every bit of air is removed and then it will go up to 20psi. So i do this for what seemed like 20 times. Finally the pump went up to 20psi and stayed there. Why do small amounts of air get trapped and never leave? It seems to me they eventually would circulate out of the system through the fountain.