A pump has a body and a pressurizing barrel. The body has an outlet tube and a magnetic check valve mounted in the outlet tube to prevent a back-flow current from damaging an impeller mounted inside the body. When closed, the magnetic check valve actuates a magnetic switch to signal external devices. The pressurizing barrel has a tank containing liquid for compensating a pressure in the body and a casing. The casing is externally mounted on the pressurizing barrel, defines a notch for receiving a circuit board and has a detachable cap to facilitate maintenance of the circuit board. Accordingly, the pump effectively prevents impeller and motor damage due to the back-flow current. Moreover, the casing facilitates maintenance of the circuit board.