Your Friend the Pipe is one of many Written Books that may be found in dungeons to help you learn how to use GregTech 4.
It's me Mario, and I will explain you how to use Pipes.
First we will talk about the various Fluid Pipes. They are simple and easy to understand. You first put a Pump Module onto the Input of the Pipe and set it to import Mode, to let it suck Fluids out of your Tank. Then we lay a Pipe to our Target.
When the Target is connected the Fluid will flow into it. The Pipe will take a few seconds to get at full Speed. If you want to measure how much is going through the Pipe, then put an Active Machine Detector on it and place a Redstone Display at it.
Now we can go to the Pneumatic Item Pipes. Their use is simple, you put an Item into a Pipe using the Output of a Machine, an importing Conveyor Cover, a Hopper or a Dropper, and the Item will search for the closest valid targets to go in.
These Pipes have a Capacity of X Stacks per second, depending on the Material and the Size of the Pipe. When a Pipe tries to route an Item, the capacity of all which are closer to the destination gets used even if it is not in the same direction.
This can lead into Pipes being stuck if there is no target, so that they all junk up, so make always sure the Pipes are large enough and that there is always a final target, if you have multiple Inputs.
You can put a Conveyor Cover onto the Pipe to suck Items out of an Inventory, when using Import Mode. To measure how much of the Capacity is being used, use the Active Machine Detector.