Today I received two Parker 51s.
Both with great smooth medium nibs! Now I know how a 51 should be

But: while flushing the black one I got ink an my fingers. I pressed the bar to suck in water into the ink sack. For some reason I got inky fingers. There came ink out of the space between the metal cage and the ink sack!

Do I have a leaky ink sack? Or is there some other reason why there is old ink between sack and cage?

I'd rinse the pen well, and then fill it with ink. If there's a leak in the sac, ink would go out the pen through the nib. Can you guess whether it's the original sac? or any sign of repairs?

There might be a leak in the aerometric-section joint too.

Update: in my eyes the pen seems to have the original ink sac. But I am not an expert with Parker 51s.

I have inked the pen with Lamy blue (my "test" ink). The pen seems to hold the Lamy blue but there is still some old ink (dark blue black) smudging my fingers as soon as I operate the pressure bar.

Is there any way to clean the ink sac/metal cage without damaging anything?

Rinse it. Put the whole pen (without the cap and body, that is) into water. You can add a few drops of dishwashing fluid. Then dry it.

When I'm cleaning my 51s I wrap the nib section in a dry paper towel (ok, it's WC paper) and shake it like a thermometer (be careful, or you'll have a 51 flying) ; you could do the same wrapping the whole pen. Repeat until you get no ink in the paper towels. Then fill the pen with ink and check if there's a leak..

Hello Juan,

thanks a lot!
I have rinsed it and got most of the ink out of the space between ink sac and cage.
I have filled it once again and it seems to work fine.