My pulley moves in and out about an 1/8" also. And I have no such problem with the belt. They all move in and out a little. I'd look elsewhere for for the problem. Unless of course the pump is leaking or making noise, then it needs replace anyway.

 

It's not leaking or making noise the idler pulley doesn't spend real easy u have to put some effort to make it spen

 

All the pulleys should spin freely without much if any effort. Except of course for the crank pulley. Like I said, make sure all the pulleys line up as best you can. Maybe some one replaced a component and its not lined up right.