Hydraulic Valve Lifters

Hello.
I disassembled my 1.9 TDI engine with engine number AVF. During reassembly, I noticed that some of the hydraulic lifters had oil leaking out and were bouncing excessively.
Question:
- Is it necessary or possible to refill the oil?
- Are the pistons automatically vented?
- Besides the assembly procedure documented in the manual, what else should I be aware of during assembly?

Sure, here's the translation:

'Thank you very much.'
Best regards, Peter.

"I remember from my BMW days that after installing the camshaft, you're supposed to wait for 1-2 hours to allow the lifters to settle before starting the engine."
It is possible that not all valves are fully closed.
But by the time you've assembled everything like that, the time is already long gone.

Drill a hole on the opposite side to vent it, and inject oil with a syringe .

Okay, just kidding. Actually, you should always put the valve lifters back in the same place where you took them out. If you're not sure where they go, let the engine sit for 2 hours after reassembly, as Steffarn described, so that any remaining oil can drain out of the valve lifters.
Don't be alarmed, after starting, the engine makes a knocking sound like it's defective. Let it run for 2 minutes, then drive for about 5 km, and the knocking sound should disappear.

Best regards, Manuel.