Ultegra brakes - caliper losing tension?

plumpy

I have a problem with an Ultegra 6600 rear brake. I'm using "right" and "left" to mean, as seen from behind the bike.

The brake works effectively when the lever is pulled, but when the lever is released, although the pads fall away from the rim (so the brake is not dragging), they do not "spring" away from the rim and the brake cable hangs slack below the top tube. I undid the cable clamp, intending to tighten the cable run, and found that the Right caliper arm was completely "slack", ie not under any spring tension at all - it simply flops up or down independent of any movement in the rest of the brake unit. I took the brake off, intending to replace it...and found that normal spring tension had returned, both caliper arms were under tension. I put it back on, tightened up the mounting bolt...and the right arm became "detached" once more, with nil tension.

What on earth is causing this? The brake is fitted on a normal frame and does not touch the frame anywhere except the back face of the mounting bolt assembly, and I am tightening the mounting bolt only to the recommended 8-10 nm.

thanks

dgunthor

IIRC there's normally a washer between the rear brake and the frame, with the bolt the other side of the frame.
Do you have the washer there?

Other thought is the tension is adjusted using the 4mm allen bolt (at front of caliper) and 10mm nut (on rear) on one side of the caliper - if this is too tight it won't allow the brake arms to 'spring' back

plumpy

Thanks - yes the normal washer is there, and no I haven't adjusted the tension. Off the bike, the calipers seem to have absolutely normal tension; when I fit the brake to the bike and tighten the mounting bolt, the Right caliper doesn't just have reduced tension or a "stickier" movement, it somehow becomes "detached" entirely, is under no tension at all, and can be flopped up and down.

I should also have said that I have taken the brake off and replaced it with another, which is working perfectly - so it is definitely nothing to do with the STI lever, or the cable or the housings.

keef66

Has the caliper ever been taken to bits?

dgunthor

Ah, ok.

If you leave the cable connected to the brake but remove the brake from the bike, does it work ok?

There is the other end of the mounting bolt that holds the caliper together but never need to touch that. IIRC if you squeeze the brake blocks together (without a rim in the way) you can get to it - may be something to do with this.

Only other thing I can think of it looking at the caliper spring itself - compare it to the front brake, how does it look?

As keef66 says above, seems strange and only dismantling a caliper can cause this sort of issue.

plumpy

Thanks both - yes it is thoroughly weird. The caliper hasn't been disassembled and I'm certain the brake worked normally when fitted to the bike and for a long time thereafter - it was only recently I noticed the cable was slack and that the calipers weren't springing out properly. Yes, if you undo the mounting bolt, that restores the spring tension to the Right caliper, with the brake still in the same place on the frame.

I can only think that tensioning the mounting bolt is distorting the brake body in a way which allows spring tension to be lost - but I appreciate that it sounds crackers!

dgunthor

http://www.parktool.com/blog/repair-hel ... ke-service

Section above covering spring tension adjustment, may help