what is this part???

[kalbert]

I doubt it, the hydraulics are designed to hold thousands of PSI, tightening a tube nut won't cause any cracks other than in the tube if it's not done properly. If you just put new lines on, and now you have a leak, chances are the nut is not tight or cross threaded or stripped and the flare is not seated or is damaged and not sealing. It is also possible that after replacing lines that were leaking you've exposed other leaks in wear parts. It's not uncommon for the seals in wheel cylinders or the master to blow after replacing a hose or line, just because they became the weakest link.