Does loading a line to its max damage it? I mean if you lift a 55lb weight on the 60lb line, does that damage the line in any way?

if you have a knot involved only very few lines can ever tak up near 100% of their breaking strain. it would probably fail much before reaching the 55lb mark, depends on the line and the knot used.

comming to the point, if you pull a line very close to its breaking strain it would loose a lot of its elasticity and remain permanently extended thus reducing its performance

Thanks. So what is the max safety limit for testing a line without damaging it?

I believe that the only way to know that is by testing the line yourself or there are some well known manufacturers that on their products state the breaking limit of the line, try to look on their web sites for such information.

if you want to test it you need to break the line to know its limits.  as regards to drag i try not to exceed 1/4 if trolling at good speed, the initial stike and the first run of the fish may be enought to cut your line even at 1/3 drag... from physices F = m a so if a is high (as in the case of a tuna) it can easily cut your line during its first run, especially if you are using braid that has no stretch

to test a line just take a sample length and test it then just throw it away. The rest of the line will be similar to that test. I dont think u need to lift with the whole line let out yes......so take a 2 mtr length and test that.

What I mean is when tying braid to line and line to lure - how much pulling is safe to test that knots hold without damaging the line?

just put some good pressure, if the knot wasn't done propperly it should give way immediately

if the line is say 100 lbs (for the sake of percentages) I would try 70lbs pressure and see if it gives.
there is another way Julian how much is the lbs(kgs) that the drag can cope with. If its a miya epoch its approx 30kg (if i am not mistaken) then use that as a max breaking strain or just above it.

always wet the line especially mono before making the knot it will make a better knot,