"What could be causing our rubber return line to continue to break?"
The main causes of rubber return lines breaking typically are related to the quality of the hose, the pressure rating, heat, sunlight (UV degradation) or even the lubricant being used.
Replacing a degraded and broken rubber return line with either a low-quality or even a used rubber line will only increase the chances of the line breaking again in the near future. Make sure you are using a new, high-quality rubber return line every time one needs to be replaced.
You should also check the pressure rating of the rubber hose you are purchasing and confirm that it is of a high enough quality and rating for the pressure of the system in which it is being used. On old lines, the print of the pressure rating can wear off, so always remember to verify the correct pressure rating each time a rubber return line is replaced.
Heat can be a big factor in the degradation of a rubber return line, which eventually will cause it to break. Depending on the quality of the hose being used, the polymer structure can begin to separate at high temperatures, leading to cracking of the rubber hose and ultimately a break in the line. If heat is an issue in the environment or process, then you may wish to consider buying a stainless-steel return line.
UV degradation can have the same results as heat degradation. Just like heat degrades the rubber line, UV radiation can also attack the polymer structure of the line, causing cracking and breaking of the rubber hose. If the system is frequently exposed to light, try covering the return line so light cannot get into direct contact with it. Otherwise, you may need to switch to a stainless-steel return line.
Depending on the formulation, synthetic oil can be harmful to rubber return lines if it is being used in the system. Glycol can be found in fire-resistant hydraulic fluids. It is well-known for attacking rubber (polymer) lines and causing micro-cracks. Over time this will continue to degrade the rubber line, resulting in more cracking and eventually a break in the return line.
By checking all of these important factors, you should be able to determine what is causing the rubber return line to continue to break.