Department of Transport. Any auto or truck technician knows this. It IS against the law and if your brake line ruptures at the commpression fitting and causes an accident or death then your insurance is VOID and you and/or whoever put the fitting in are responsible for the lawsuit that's sure to come.
As I said, look on ANY car or truck and show me a commpression fitting (from the factory) on any critical hard line, brake, gas, oil etc. You won't find one because it is illegal.
As for substantiating my claim, well, I'm not sure where exactly the law is written but if you want I will spend some time looking for it. Just look up "brake lines" AND "compression fitting" and you will find almost every web page say's NOT to use them, they are weak and leak or rupture under 2-3 thousand pounds pressure.
Kel
Licenced Auto Tech