I have replaced mine with both hard lines OEM and done flex lines.
the unions are a biach if they are all rusted up.
after doing the flex line approach I would suggest it's way easier than hard lines. you will probably need new brackets/bolts/cushions, clamps, unions, o-rings. it's not killer expensive but... it not free either.
here is what I did to convert mine to flex lines
https://www.toyota-4runner.org/3162709-post2.html
I would suggest doing o-rings for sure, they will leak once you take them off. those old o-rings will be hard as rocks and probably be in bad condition.
this is another view of what the completed to flex line looks like
https://www.toyota-4runner.org/3356856-post139.html
the one part of the project that is hard is locating the tube nuts and doing the flare/bubble end on the unions. it's not hard but you need a good quality brake/tube flare tool. not that crappy one you can loan out from auto parts store.
if you have specific questions I'm happy to help.
if you decide to go the hard line route? put a little how I did it here.
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