After snapping a front line midkiteloop and breaking 4 ribs in 2009. I suggest if in doubt replace them. Check for knots and any fraying especially near the pigtails. Snapping a front line is serious even doing little jumps especially on flatter bridled kites.
When I was working in a kiteshop we tested a few lines from different companies when a customer left a broken one. I weigh 83 kgs and we attached an old bar and put our weight on one line. Some were disturbingly easy to snap, lines that were 1-2 seasons old.
Since my 10m RPM was stolen in 2010(kite only). I have used 1 bar for 5-8-10m combo for the last 6 seasons. The slingy bar is heavy, has below the bar depower and all that, the swivel eventually siezes up and if you dont treat them right the depower rope wears out........
However it is one of the most over-engineered pieces of equipment in kitesurfing history. I rarely drop my kite after many seasons, I edge rather than work the bar and I always roll my lines up and leave them on the kite for a break. My 6 year old bar looks like a new demo bar. 400lbs fronts, 200lbs backs. The only thing slingshot ever made that was bombproof was the bar and lines.