Have You Tried?
Keeping your brake cables well lubricated is a great way to extend their serviceable lifespan. There are a few ways of doing this and you might even be able to restore your old cables using these techniques!
If you notice your trailer brake cable is stiff and not free to move it can effect the performance of your trailer brakes. The best thing to do in this situation is remove the cable and suspend one end (just as long as one end is higher than the other you should be just fine). Drop some thin oil into the suspended end and allow this to soak through the cable. Repeat this step a few times. Now try moving the cable in and out a few times - It should be noticeably easier to use!
This is a great technique to use with trailers that have been left unused for a while, but its also great for brand new cables. Putting some oil on either end of your brake cable will help repel water and keep debris out of the inner length of cable.