• First, set the caliper on a stable wooden surface. Then smack the nipple directly on the end with a large hammer. That will shock the threads into breaking free.

• Secondly, heat the body of the caliper around the nipple (not the nipple itself). Use a small torch, such as propane, that allows you to focus the heat. The body is aluminum alloy and it will expand more rapidly and break free. 

Causes and Prevention

This situation is created when the wrong wrench is used to bleed the caliper. DOT4 brake fluid always creates corrosion, so the repairman should always expect a stuck nipple from the outset. To loosen these a tight-fitting 6-point box-end wrench must be used. The jaws of an open-end wrench will simply spread and loose contact with the "flats". A 12-point box-end doesn't have enough contact area, it must be a 6-point !!

Automotive brake wrenches for steel lines

There are special automotive "brake wrenches" available with a slot to step over rigid brake lines. Even though they are slotted, they are still 6-point box-end. The Airhead doesn't deal with a lot of steel lines, especially when when it comes to "bleeding", so a long shank 6-point box-end works great. An offset neck can also be useful. 

Offset brake wrenches for bleeding

Rounded or distorted bleed nipples should always be replaced while they are still able to be grasped and removed.

Hope this helps

@wobbly thanks. I’ll give that a try. 

@wobbly Turns out this didn't work. I could get no movement at all from the brass fitting, so I took it to my local brake shop. They had to cut it partially off and weld a nut to it, after which they were able to get it out. 

Glad you were able to salvage the caliper. Your issue was a great ad for using 6-point wrenches.