I bet the end of the pump has burnt out. On the surflo pumps, you have a device on the end to cut the current and stop the pump pumping when the trigger has stopped being pressed.
This happened to me once. I checked the fuses, the current then the pump.
If you take the pump off the machine, you will see a square like thing held on by four screws if i remember. Be careful to note the order that it comes apart. It will then become apparant how it works. Inside this square assembly there is a slimline micro switch; the type you find in washing machines. This usually burns out, and not the pump.
How the thing works is thus. When you first turn the pump on, it rumbles for a bit, then stops. What it has done, is to draw water from the tank, filled the pipes with water and the pipe on your wand or handtool. If the trigger is not being pulled, then a plunger in the pump body hits a coorresponding piece in this device to cut the power to the pump, otherwise it would carry on pumping and burn out the pump itself.
If the microswitch is faulty, this can happen. I hope in your case it's just the microswitch. If you took the pump to a specialist, they would probably renew the whole pump when theres no need. Check it first by smelling the end.
Hope i aint confused you with this explanation.
Dave