Disc brake rub - but not all the time

[foxyrider]

Before we go any further i'd like to state that i hate disc brakes!

Now that's out of the way, the issue. Last week i finished rebuilding my CX/touring bike, Its had new frame bearings, all the consumables have been replaced, mudguards and rear mech replaced, its looking pretty good and even made it onto GCN this week. The only fly is the braking, its never been great from new, it had HY/RD's on for some time, i upgraded the front rotor to 180mm but they were never great so i reverted to the Shimano cables that it came with. I had got to a point of acceptable performance before the strip down, no noise to speak of and as long as i kept on top of the pad adjustment (at least once a week), stopping power was similar to old school single pivot rim brakes.

The first couple of rides after the rebuild suggested i'd got everything dialled in again but the last two rides the front brake has devoped an annoying quirk, vis it drags on the disc at slower speeds, a dab on the lever clears it but it soon returns. I've stripped the brake down, changed the pads, checked the flatness of the rotor, realigned the brake on the fork and voila, no drag/rubbing. A couple of hours into todays ride and its back.

Any ideas? The first two rides were bone dry but the latest have both been a bit humid although actual rain has been scant. The pads are a mixed compound. The rear brake has no issue beyond poor performance.

[Cyclothesist]

Sticky cables maybe? Have you tried cleaning and lubing the brake cables or replacing them (inner and outer) if that doesn't help?

[tim-b]

Sticky piston?
Float in the hub?

[Cyclewala]

As above (cables) or sticking piston. With the pads out, WD40 or similar around the circumference of the piston. Press the brake, push piston back with flat head screwdriver. Repeat a few times to flush out any dirt and free up piston.

[gxaustin]

I found that compressionless brake cables made a big improvement to my HYRD brakes. They are constructed like gear cables instead of being wound like a curtain spring. It sharpened them up no end.

[foxyrider]

Just to restate, its got new cables and compressionless outer, its not a cable issue. Oh and its not the HyRd brake but a Shimano single piston.

I will take the brake apart and have a go at the piston, it could be that although it seems to work okay.

[geocycle]

Are the shimano cables 1.2 mm whereas the outer are designed for thinner cables eg Clarks?

[Cyclothesist]

Another potential cause is heat related disk warp. That happens after a bout of prolonged gentle braking. I would get it on a single piston Hayes CX mechanical brake. I don't get it with Spyres (on the same rotor). When the disk heats up it can warp slightly until it cools down again. The static inner pad on a single piston brake sits very close to the rotor so even a tiny bit of warping causes rub. Any debris stuck to the surface of the disk or pad narrows the gap further and causes audible rub. A quick dab of the brake clears the surface debris and the noise goes.

[foxyrider]

Are the shimano cables 1.2 mm whereas the outer are designed for thinner cables eg Clarks?

neither, they are all Campagnolo

[Brucey]

discs really need to be perfectly straight. Any 'high spot' is likely to be self-reinforcing if it starts to rub; the rubbed surface heats up slightly, expands, and this makes the warp a bit worse and so on.
Discs vary considerably in how straight they are to start with and how likely they are to stay that way; with a mechanical disc brake you can at least adjust the running clearance. What brakes, discs and pads are you using?