overtightening will definately cause the compression fitting to leak. Once it has started to leak due to overtightening it is ruined and will never seal. If you have slack in the line you can snip off the ends and get a new ferrule and try it again.
One alternative would be to get a braided steel line (best) or a copper line. However the copper line can crack & can be over tightened just like the plastic line.
http://www.summitracing.com/parts/ATM-3229/
http://www.summitracing.com/parts/VDO-150856/
Z.