Cistern mechanism help

[grahfcard]

I'm having trouble with the three part machine puzzle in the cistern level. I got the first two parts, with the weights and the pressure boiler mechanism, but I can't do the pipes one. Problem is, I can't find the fourth pipe: I got the two in each of the weight puzzle rooms, and the one in the pressure mechanism room. I fixed them to the wall in the final chamber, but there's one missing. I've looked everywhere, methodically sweeping the rooms, but I can't find it.
It's not an item you can take into your inventory, so it has to be in that map, and not in another one of the Cistern level.
Anyone know where the fourth one might be?

[Spookie]

You don't need 4.

Your... Inputs? At the end should be bare, use one of them to cross over the middle pipes.

[grahfcard]

Hmmm, that crossed my mind, but how do I pull them back out? I can't interact with them once they're in.
edit: Oh, I see now, that's because they were put in the correct way and the puzzle was actually solved. It was a bit confusing, and I thought I'd done something wrong.
Thanks for clearing it up

[RoadCrewWorker]

You're not supposed to close all holes, the idea is to mirror the connections on the opposite wall (that is, link up the same openings). Every other puzzle in that segment is based on symmetry as well.

[Dark Knight]

You're not supposed to close all holes, the idea is to mirror the connections on the opposite wall (that is, link up the same openings). Every other puzzle in that segment is based on symmetry as well.

yes you exactly right about rule of Symmetry!