Storm door lock spins

Thanks for posting. Does the thumb lever on the inside pull work to lock and unlock the door? If it does not, try removing the handle set from the door, and then attempt to 'throw' the door's internal mortise lock with a slotted screwdriver. If it doesn't budge, the mortise lock is broken and needs to be replaced. See our 40-191. 

If the thumb lever works, and if the mortise lock operates fine, then you may just have a damaged key cylinder. Older versions of this handle set (the Pella 40-199) use a variety of different key cylinders, so I can't say for sure what you'd need. Either the 40-123 or 40-129 might work, but ultimately you'd need to compare your old hardware to either and see if one of these match. Let me know what you find.