i had the same problem this was a nightmare job for me there is a connecting piece in between the pipe and the block, it like a nut with threads on each end everytime i went to tighten up the nut on the pipe it would work the other loose, in the end i had to remove the pipe where it connects to the block tighten up the other nut first, then carefully fit pipe back on, i think i had to hold the pipe with something whilst i tightened it back up to stop the pipe twisting and working the connecting piece loose hope this helps and makes sense