Few tips for rusty seized bolts:
1. The best tip for breaking the bolts (not the head of the bolt!): use a
gas torch (a lot of videos about it on youtube). Heat area where bolts are siting, NOT the bolt! With heat, metal will expand and it will make the job much easier and safer for the really seized bolts which you can literally brake. Try to avoid heating the bolt, because it will make it expand too. The heating works much better than any anti-rust spray. You should heat for about
5 minutes the whole area around (or at least where you can reach, because are of the bolt located behind the brake disc).
2. If you don't have a long enough breaker bar (or usually there is not enough space) then you can use a
jack under the wrench (try to avoid using socket wrench because you can brake the springs (or whatever is inside). The jack will not help on the left rear side, because there is not enough space from the other side and you will lift the whole wheel with suspension. Always be sure that you are going to move the bolt anticlockwise. With the rear left side, you can use a
ratchet strap: connect one side to the part under the rear bumper (NOT to something fixed near the car, because you will move the whole car from your jack/stands!!!) and the other side to the wrench.
Be careful with your face and hands, if it will slip somehow,
it can hit you!
I had a super seized bolt and I put a lot of anti-rust spray on the bolt for 2 days and then broke one 1/2" - 3/8" socket adapter with the ratchet strap method, and only after that I heated the caliper carrier bolt area and that bolt came out like freshly screwed!
3. If your seized bolt moved a bit but doesn't go any further, then try to move it there and back, eventually, it will go more and more.
4. Try to
avoid using 12 point wrench, use 6 point socket. 12 point is the way to round your bolt head.
5. If you change brake pads,
make a full inspection of caliper pins and change grease (clean old) with new
silicon grease (not copper, because it's bad for rubber sleeves!). Also check all sleeves and other rubber things, if there are any holes, better to change it, otherwise you will have problems soon. Caliper pins are very important parts of the brakes and if they don't work, then your new brake pads will not make any difference!
6. With rear calipers also check that handbrake mechanism works fine and easy. If it sticks or moving very difficult (you should be able to move it with one hand), then probably it's time to clean and change rubber parts (then you have to remove it and make full rebuild with a repair kit).