Maybe I'm too anal about brakes, but this is why when my brakes are about 5-7 years old and needing a pad change, I simply dive in and replace the ENTIRE brake system, including hard and soft lines. Yes, it's bucks, but it sure eliminates the frustrations kinda like you're going through now!

Of course, you say this is a customer's car, so they won't want to cough up the ching to do it, but for my personal vehicles, this is what I always do.

Money well spent, in my opinion!