A cheap solder sucker I got some time ago lost the ability to pull a vacuum. The O-ring on the plunger failed to made a good seal. the slightly bent shaft may be partly to blame for this. The easiest solution would be to use a larger o-ring but I didn't have any suitable replacements. I then tried a few things on the O-ring to cause it to expand including some isopropyl alcohol which failed.
I then thought about putting some material in the space that the O-ring sits in to stretch it out to make a good seal and for the packing material I turned to this . . .
Yup, floss. I wrapped it around in the groove that the o-ring sits it. I had to go back and add a bit more as I didn't add enough the first time around.
I then put the O-ring back on and greased it with a bit of Vaseline. After all of that I re-assembled it and it once again can pull a vacuum.
And it's back together!
Side note: I wish that I had a proper desoldering gun/station (desoldering plated thru-hole components can be quite a pain) but can't justify the cost of one to use for occasional jobs.