brake line opinion

To put my 2 cents in on your question, yes I sure would go ahead and replace them. They may look to be in decent shape from the outside, but how do they look inside? That's one of those questions that you never quite know. They very well may be ok inside but that rust, slime and scale you found in the bottom of the master is surely caked and crusted on the inner walls of the lines. If it's within your budget, you should slip some nice clean new ones to go with your other fresh components. That's just my own personal opinion. :-)