Chesterton

I am in the process of building my new road bike (Cycles Toussaint Velo Routier). This is my first experience assembling a bicycle from parts, so it is an ongoing learning experience.

The bike came with new Dia Compe 750 centerpull brakes, to be mounted on brazed-on posts. As part of the "ongoing learning experience" I overtightened the mounting bolts at first, and managed to mangle the plastic bushing on one of the brake arms. In the process of removing and reinstalling the arm in an attempt to sort out an unequal spring tension issue, the two halves of the bushing have been mangled up past the point of good repair.

Short of buying an entire new brake assembly, I was wondering if I can replace the bushing itself. The local community bike workshop has a bin of old Dia Compe and Weinmann centerpulls. Are the bushings in these old brakes interchangeable with the current production?

When installing these centerpulls on brazed-on posts, should I be greasing the post as well as the inside of the bushing/brake surface? I wasn't sure if the bushing is supposed to be dry on the post to discourage it moving, and the brake rotates on the outside surface of the bushing. But then I wonder if greasing the post would have helped prevent damage when installing and removing the bushings from the posts?

Are the mounting bolts only supposed to be snug? Too tight and the arms will bind up together, but too loose and I suppose there would be unnecessary wear on the pivot points.

Finally, I'm having a terrible time with the spring tension on the rear brake. I almost suspect a manufacturing or design where the left post is just slightly too far counter-clockwise, placing the spring's anchor hole a little too far to the left, reducing the spring tension. As it is, when I pull the straddle cable up, the left brake pad contacts the rim while the right pad is still a good 1/2" or 3/4" away from the rim. I am planning to order the Tektro straddle cable hangers that have screws to secure each side of the cable, and I'm hoping that will compensate for the uneven spring tension. Is there anything else I can do to adjust these brakes?

Thanks for your help!

WizardOfBoz

First, congratulations on giving bike assembly a try.

About your question, though, I'm confused. Diacompe 750s are made with a main bar that the arms bolt to. They are classic centerpulls, not cantilevers and they don't bolt to studs on the frame. Is there a modified form of the 750s? Or do you have another model, like the 990?



https://static.modernbike.com/Produc...0418040916.jpg

Chesterton

The Dia Compe 750s and other centerpulls can mount to frame posts. The arms just pivot on the posts of the fork and seat stays instead of on the backing plate that they use for center bolt mounting. I believe in theory there is less flex when the brake arms are mounted directly to the frame instead of on a plate.

bertinjim 

Chesterton-

The bushings from Dia Compe/Weinmann centerpulls should be interchangeable to allow you to replace the damaged one. Pre-prep the brazed-on pivot posts by taking fine sand paper or emery cloth and lightly twisting it over the pivot. The post should allow the brake arm to slide on easily and rotate freely on the post with negligible resistance. Take the brake arm off, grease the pivot post and the interior of the brake arm (use a q-tip for that) place the spring into the hole on the pivot post and reassemble. Add the pivot bolt and tighten down snug. Once finished, the arm should move completely freely and not rock on the pivot post before the final hook-up of the return spring.

Regarding the uneven rear spring tension, you might want to try tweeking the rear by stretching or compressing it to add or decrease tension. Cannibalize an older Dia Compe and try different springs as well after you strip the caliper for the bushing you need.

WizardOfBoz

Thanks, Chesterton.

Chesterton

OK, so basically grease everywhere, on both sides of the bushings?

I have been trying to stretch the left spring will perhaps some progress. They don't look quite as asymmetrical now. This evening I'll rummage through the bin at the community workshop for an old Dia Compe to scavenge bits from, and I ordered the Tektro cable hangers in hopes that this will further compensate for the uneven spring tension. The more I look at it, I really think this is a result of poor manufacturing tolerances in terms of the positioning of the brazed-on posts. There's a lifetime warranty on the frame against defects but having almost finished assembling the bike, the idea of pursuing a replacement frame sounds like more hassle than its worth (especially if this defect is present on all the frames and not unique to mine).

bertinjim 

Chesterton-

Grease the pivot post and the interior of the bushing where it will seat on the post. That will give a grease to grease interface.