These days it is better to just replace all the rubber lines and be done with it for ten years. They usually fail internally where you cannot see it.
Steel lines should be replaced if they are rusty. I like to pull the nuts back, put some Neverseize on the tube and slide the nut back into place. Then there is less chance of the nuts seizing to the lines over the coming years. The lines that seem to be worst are the rear lines on the caliper. Easy to bend up new lines.
I too have seen jelled brake fluid. It looked like semicured clear silicone sealant. I was working at a VW dealership at the time when a dealership owner, from another location, and I were discussing brakes with a mechanic. This owner said they never changed brake fluid at their dealership, so our mechanic showed us a master cylinder reservoir that was full of jelled fluid. That stuff could not flow, a car with that in its system would not stop. Eric and I have seen hundred of calipers with rust inside. They are a real pain to clean. Just disassembling a rear caliper piston to clean the adjuster guts can take an hour. The rule of thumb is to change brake fluid twice a year, and that is cheap insurance. I have said many times that draining a caliper is about the only way to get all the old fluid out. If you look at the caliper you can see that the brake line goes in very near the bleed nipple. The fresh fluid has no need to even flush the inside of the piston nearest the brake line let alone flush the piston on the other side of the caliper. At best, bleeding calipers without draining them is only a line flush. The best results are obtained by flushing the lines until fresh fluid comes from the bleed nipples, then draining the caliper, refill and finish bleeding. Yes it is more work, and yes it is far more effective.