The NAPA guy is not driving the truck, easy for him to say.
But take his word and choose to do nothing. Good luck when you have your family in the vehicle and you need to stop in a hurry. It is possible for BOTH lines to burst at the same time and they do. You should ask him about the rear brake backing plates as well. So when you blow both lines and if you are lucky enough to have the time and forethought to try and use the "Emergency" brake, and the backing plates collapse, then where does it leave you?
Kind of funny how you just are a going to be "cheap" with a KNOWN safety problem and will take a wait and see attitude with all of this. You may be lucky and have a light come on but maybe you won't. You going to drive the truck to the shop when the brake line(s) fail or are you also going to pay for the tow as well? Oh, you have AAA or your insurance may cover the tow so who cares? You prepared to be inconvenienced? Oh, your retired so you have all the time in the world? I am sure there is every excuse in the book.
Bet you would not have bought the truck if you knew all of this or if you were aware of these issues you might have planned that this is just the cost of doing business and factored this all into the buying decision for for the truck.
You know you could have gone out and paid over $50k for a new truck and you would not have these problems, but you would have spent over $50k!
I just do not get being cheap when it comes to safety, but this is YOUR choice. But you may also be compromising someone else's safety on the road as well.
Answer this question, would you allow your wife, mother or daughter to drive this truck knowing the failure potential?
Good luck to you and anyone on the road around you.