I’ve been using an older used MM722 for the last two years as part of the Loop automated insulin dosing system. I don’t know the exact age but I think it’s about 10 years old. Pumps of this era are pretty rugged and can take a lot of daily wear and tear.
One of the areas where this pump can show damage is with cracks across the battery cap threads. This happens when the user over-torques the battery cap. The pump will operate with small cracks but it will become a problem if the cracks cause a chip of the pump body to fall off. I’m aware that even then, some users have been able to repair and return to service.
Also check that the bottom outside of the reservoir tube and make sure that the round end-cap is permanently affixed and is slightly recessed into the pump body. There have been failures where this end-cap pushed out past the bottom of the pump and an unthinking user pressed the cap back into place and cause an overdose of insulin. This defect can be repaired, too.
I would make sure that the pump delivery screw moves up and down well. This can be observed by delivering a prime and then rewind command. You can’t do this without the pump thinking that a cartridge filled with insulin is installed. You can simulate this just by issuing a prime command and holding a pen or a small finger to provide some back pressure against the screw.
If you’re on Facebook you may want to join the Looped group. There are many members experienced with buying the old pumps needed by do it yourself systems. Good luck!