Age will do that. Most manufacturers recommend flushing every 2 years...
Mixing silicone and acid-base brake fluids will tell up too.
Dot 3-4 will not mix with dot five.
Read the cap!
Clorine based brake cleaner works best, just don't get it on hot engine parts... Instant mustard gas
I would recommend disassembling and washing
at least the master cylinder. If not the calipers too.
Compressed air helps clear lines...
Make sure the bleed back hole is clear, I often drill them out with a pin vice by hand. But then you need to hone the bore...
Be smart to have a rebuild kit(s) on hand, if needed. But I have gotten away with reusing rubber seals, if the fluids did not mix...
My favorite bleeding method is to use a big syringe and push the bubbles up from the bottom.
They sell them as brake bleeder kits, but I have used children's medicine syringes in a pinch.
Replacing rubber lines with nylon core "braided" line increases brake effect greatly!
Once the pads get soaked with brake fluid from a leaky caliper, there's not much for it but to get new pads.
Make sure and clean/grease the caliper float pins! Anti seize mork great here.