I suffered a similar thing on my Interpump pump so if you have an interpump pump this may help.
On the pump there are 6 large nuts, (unless you are using some for addons) under them nuts there are some small plastic things, undo the nuts and with some long nose pliers take each little plastic thing out and check it and put it back in.
They get stuck from time to time and the guy at Dual Pumps told me these stop the pump from drawing water.