[wingnut60]

Shurflo pump? 5.7 model? There is supposed to be a built-in check valve in the pump end that keeps this from happening. Happened to me--had to replace the pump. I think Shurflo has had problems with this model and no longer make it.
There might be a shut-off valve in the plumbing somewhere that will take the pump out of the system--the water supply is feeding back thru the pump into the tank, so if you close off the line, this can't happen. If you don't use the pump, then just disconnect the line and plug it off.
In my case, some quick on/off of the water pump worked for awhile, then I took the pump out and dissaembled the pump end off looking for grit, then put it back in and it worked again for awhile. Finally, tho, had to repalce it.
Joe