this brake uses a traditional brass bush which turns directly on the boss
there are only two main ways this brake model can get stuck;
1) that it is seized solid and doesn't even rotate on the boss any more
or
2) that the brake arm still rotates OK but there is a burr on the end of the boss which is preventing the brake from simply sliding off.
The latter can arise if the bolt has been overtightened, for example.
[FWIW the nominal wall thickness of the boss above the screw threads is only 0.5mm; you cannot expect to tighten this M6 bolt like other M6 bolts; if a canti brake has a dummy (integral) boss it is possible to tear the boss apart because the boss sees the tensile load generated by the bolt. With brakes like yours you just weaken the boss and raise a burr on the end of it, if the bolt is overtightened. This bolt is best tightened to a modest torque with threadlock on it to prevent loosening.]
In most cases if you add oil and keep on working the brake arm on the boss, it will eventually take the burr off (by abrasion) and the arm will come free. If you try and use a puller the boss may just fail in tension (as per comment above), so for goodness sake don't use brute force unless you have some means of replacing the boss up your sleeve.
cheers
