Cutoff direction control via shader or script

Hi guys,

I am trying to show fluid flow via cutoff and transparency on a pipe. I wrote my own shader and its working for filling the pipe. I just reduce the cutoff and the pipe fills. The problem is to empty the pipe in the same direction which the pipe fills. Now the pipe empties in the opposite direction and it looks weird. It looks like the fluid would flow backwards.

Can someone tell me how to reduce the cutoff from the same direction which it comes from? I don't care if via shader or script.

sry for bad english

Ty