You are right.. the lines from the body to the doors are solid.. However you can only bend a solid tube so many times and over time they do crack.. This causes the system to leak down .. Because they are solid most people just do not look at them.. and cracked ones are common.. I have to wonder if you do not have two problems.. when the door system leaks down nothing works well.. and you can hear things that can not quite work because of a lack of vacuum.. Its a minor job to check the lines where they bend at the doors..

Ken