For the return lines I use 4mm polyurethane hose, which is typically used for compressed air. It is very cheap and you may find it in any hydraulics shop. I fitted it more than a year ago and it is holding up great. It doesn´t crack and it doesn´t go stiff over time.

As for the air leak, it seems that everybody who remove the pre-chambers end up with a leak upon reassembly.
I guess you should pull the chamber out again (remember to remove the GP first), make sure all the mating surfaces are totally clean and put it back together.

Good luck
gonzalo