Corsa D 1.3 CDTi 2nd Slave Cylinder Leaking

[Faxo]

So I've a Corsa D with a fault, I replaced the 5spd gearbox and also swapped the slave cylinder from the old box as it was only 5 or 6k old and apparently is an updated part. I bled the clutch, and it leaked all it's fluid out the bottom of the gearbox, in one big puddle

I removed the box again, and replaced the slave cylinder, this time with a brand new one, which was identical

Problem is, after bleeding it, it slowly drips from the bottom of the bell housing, around a drip ever 15 seconds, until it's dripped dry - not the massive monsoon it did before

Surely a new slave can't leak the same as the original did?

Is there something I'm missing with these gearboxes?