With a gas one when you release the trigger the pressure goes up instantly. An unloader is present which vents the water back to the intake. In a circle. But each pass heats the water. Finally it melts some of the plastic/nylon parts in the pump. The unloader can be part of the pump or plumbed externally.
Now if your in the mobile business and come with a large water tank, they vent the unloader back into the tank, so you could probably go to lunch and leave it running. But I doubt that is what your are doing.
You can buy rebuild kits for most pumps. Assuming the housing did not freeze and crack.