I have had brake issues on older vehicles(pre 1990) that I felt were related to water getting into the system and causing corrosion. It does seem that the newer vehicles are made better and this is not as much of a problem. I used to flush the lines myself just by getting someone else to mash the brake pedal while I cracked the bleed valves and bled out dirty fluid replacing it as needed. Cost was just a few dollars and doesn't take long. I like the suggestion of opening the bleed valve while compressing the caliper for new pads and then putting new fluid in to replace what you let out. It never hurts to look at your brake fluid and make sure that it looks clean while under the hood for other reasons.