Play the safety card. Show your safety director how the design makes it likely that the air lines will pop off while driving the truck. Make sure that you document your concerns in an email and get his reply via email. If he won't reply in writing, then summarize your verbal discussion in an email and send it to him (cc his supervisor on the summary email) for documentation purposes. Most times when you put something in writing, you put someone on the hot seat to address it.