Well, I don't know much about retrosigmoid, since that wasn't really an option for me, but from what I understand, and don't quote me here.. I'm not a doctor and I don't play one on tv... but.... once the "seal " (the dura), which is the covering of the brain that keeps all the fluid inside is cut, it will no longer be a closed system in there - I guess they can stitch it back together, but it doesn't actually re-seal itself so it's not water tight anymore.
If it does leak at the repair site, your body will respond by producing more CSF or increasing your intercranial pressure. If the pressure gets to be too much, the "pop-off" valve would kick into action and would leak fluid to reduce the pressure. It's your body's way of keeping things the way they should be.
Now, like I said - don't quote me on the technicalities here - but this is they way I understand it. Who knows, maybe my doctor just isn't as good a tailor as others and doesn't sew very well!
Lori