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I have managed to repair 10yr old diamondback hybrid as different parts fail. I even disassembled the rear wheel to replace few spokes. But I cannot figure out how to disassemble the rear v-brake arm. (I already replaced the front.)

The complete brake arm

In the second image you can see the regular bike bolt on the left. When I unscrewed that the arm came off. But looks like the bolt doesn't directly connect to the frame and there is something else (a boss?) between the bolt and the frame and that came off with the bolt.

Closeup

The new brake (Shimano Mountain Bicycle V-Brake - BR-T4000 - Rear) is exactly like the old one, but does not have this extra piece. So I need to dissamble that from the old brame arm and reuse.

So far I have tried soaking in wd-40 and also freezing the whole thing and then heating the boss(?) but neither worked.

Also noticed that this piece is tubular and has threads inside. A regular bike bolt fits in there and probably it goes all the way. So screwing something into that doesn't give me much grip either because it turns the same way.

Any ideas other than 'take it to the bike shop'? Thanks.

The reason I need to replace the brake is that the small metal rod which pushes the brake away during releaseing the brake broke.

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  • Some brake bosses have flats above the threads to put a wrench on just for this purpose. It's hard to tell from your photos whether these ones do. If so, a wrench on there and an allen wrench on the bolt should be able to break them apart. Jul 31, 2017 at 17:16

3 Answers 3

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Looks to me like you've managed to unthread the brake boss from the frame. That's not part of the brake lever.

Normally you'd undo the dome-head silver bolt visible on the left side of your photos.

If you stick a 5mm allen key/hex tool in that bolt, and mount the allen key in a vise would be a start. Then you need to grasp the thread and undo it without damaging the thread.... that's difficult.

One "dirty hack" is to find two nuts of the same thread and internal diameter. Thread them on fully and cinch them up against each other as hard as you possibly can. Then put those two nuts in your bench vise tightly, and use a long arm 5mm hex tool to lever it off.

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    Or screw it back in with Loctite, wait a bit and hope that this time only the silver bolt comes loose?
    – Michael
    Jul 31, 2017 at 10:49
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    @Michael Pete specified red loctite because its medium strength, and should not come undone with any hand tools. Normally red needs some heat to loosen it, order of a couple hundred degrees. Brake bosses won't get anywhere near that hot. Blue "low strength" loctite works fine on bike stuff generally, but brake bosses shouldn't unthread ever.
    – Criggie
    Jul 31, 2017 at 11:03
  • Thanks everyone for the ideas. Bosses of the both sides came off when I tried to unscrew the bolts. Bosses seem iron. So probably there is some rust. I looked for bosses online yesterday, but need to figure out the right size. There are no flats above the threads. I will try the lacolite trick and report back.
    – Aelian
    Aug 1, 2017 at 3:14
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    Lacolite trick worked on both sides. Tks.
    – Aelian
    Aug 16, 2017 at 2:11
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That is the brake boss. It should not come out easily

Instead of trying to save it, just buy a new brake boss and fit it using red loctite (ie the medium strength one)

Clean the fork threads first according to your threadlocker's instructions

Consider replacing both brake bosses. One has come loose so the other might too.

Brakes are kinda important.

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Even though it should not come out easily, occasionally the brake boss will work its way loose. Sometimes you can fix this problem by screwing the whole thing back into the frame and tightening it a bit. Then when you unscrew it again the brake bolt rather than the brake boss will come free. If that happens your problem is solved!

Make sure that the brake boss is secure and doesn't have play. That would suggest some damage may have caused the loosening.

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