Seized brake caliper nuts Servicing a mate's bike. The brake caliper nuts were already rounded off. Tried all the hints to remove without success.

Agreed to saw off the calipers as new ones were needed anyway. Got this done and removed all parts apart from the nut. Both the front nut, inside the fork and the rear in the frame are currently immovable.

Soaked in wd40 and tried screwing in a bolt from the front and tapping to loosen - without success. Any further hints or advice??
I'm guessing your brake nuts look something like this, except they're completely recessed in the frame and all you can see is the hex socket on the back



Good news is there is no threadded part here that connects to the frame.  They will be rusted/stuck in place, or there's some possibility that the installer used some kind of thread-locker product and its come out the front and set onto the frame/fork's metal.


I'd start by removing the wheels for easier access.
Then hang the bike so the nut is vertical - start with the back one cos its shorter, so the seat stays are horizontal.
Get some blue-tack or something similar, and bog around the hex socket to retain the oil. Arrange a rag to catch any drips.
From the brake caliper side, squirt in some penetrating oil, like Liquid Wrench, or another brand.  CRC and WD40 are a poor substitute for the correct stuff.  
And then leave it for a couple hours.  Come back periodically and give it another squirt.  (you can also do the front one, but the angles are all wrong so it probably drip straight off)
Next day (or longer, I've done this for a week on some car stuff!)  fit a longer bolt and thread it into the nut from the top.    Tighten till it bottoms out on the inside of the hex socket.  If it pops out the other side fit a nut as a locknut and tighten that against the underside of the stuck nut.  
Then try and twist the stuck nut loose.  Don't shear the temporary bolt!  No cheater bars!
If that doesn't move it you're definitely facing some goop.  Some of the thread lock products weaken under heat, so depending on your frame you can try some gentle heat from a hair dryer. Don't exceed "comfortable holding temperature" on anything made of carbon or aluminium - it shouldn't get any hotter than "summer's day sunlight"   If your frame is steel it can take a bit more heat but be careful.
Finally, try a sharp tap on the end of your temporary bolt.  I've had excellent results from using an automatic center punch.  You'll want the brake bridge well supported.




I thought of suggesting an impact gun with a suitable 4/5/6mm hex bit, but there's really nowhere to get that into the back nut.  The front would be easier cos you can turn the bars to increase access.   Risk here is stripping the hex socket.

If you're feeling creative, building a puller from scrap may be a good solution.  It depends if you have the nuts that allow a bolt to match the thread to enter through the hex socket.



Last resort is to know when the bike's beaten you.  There's absolutely no shame in taking it to your LBS and asking for help.

And do keep your mate in the loop as well, let him make the final decision on what to do   - its one thing to damage your own bike, its an entirely different thing to damage someone else's.