What causes the nylon line to break off? There are several reasons for broken nylon line. 1.As nylon line becomes old, it will dry out and become brittle. Try soaking the nylon line in warm water. The water will restore flexibility reducing the amount of breakage. 2. Cutting too closely to hard objects such as concrete or chain link fences with the middle of the line instead of using the tips of the line. Heat from friction can built up inside the trimmer head causing the nylon line to become weak where it exits the trimmer head. 3. A Poor quality nylon line will not dissipate heat efficiently and cause the line to break prematurely.