Do a thorough inspection of the back of the master cylinder to see if it's dripping fluid down the front bulkhead and underneath the carpet, and of the slave by removing the dust cover from the pushrod end to see if fluid drips out. It may be that one of them is leaking and pulling air. The braided flexible line is standard to allow for engine movement. After insulating, if you have no more problem, then it's heat, but a Dickman slave is a wonderful thing.