Try this – pump up to 75psi (though that might be too much pressure for the seals), release the pressure, by activating the lever/remote, pump up again, release etc until the pressure equalise and releasing the level doesnt cause the pressure to drop any more.
This worked for me once (unfortunately I then trashed the post by over tightening the 10mm nut…)
2Pure’s customer service is very good (it has to be!). If it’s a Joplin 4 then it’s probably under warranty, so send it back for repair. You could cheeky and ask about a Kronolog upgrade.
But, I’ve been in your position and a £170 Reverb from Germany was too good to resist – you’ll get half that back for a second hand Joplin too.