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I checked my disk brakes for the first time last night (i've never had disk brakes before). The front ones were definitely worn out, and I replaced them. They work great, now.

With the rear ones, I decided that they still had quite a bit of life left in them, and put them back in, rather than replacing them. Now, they hardly work at all. They were working fine before.

I think that I was careful not to get them dirty while I took them out and looked at them, but I guess that it's possible that I got some dirt on them while I took them apart and inspected them.

The other possibility is that maybe the pistons which push the brake pad onto the rotor got pushed back too far out, when I was putting the pads back in, and so now they're not pushing the pad onto the rotor with much force.

Is this a known gotcha?

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    The pistons should be self adjusting, so pushing them back in shouldn't cause any lasting issues. How does it feel when you pull the rear brake, is the lever coming all the way back to the bars?
    – Andy P
    Aug 14, 2018 at 11:34
  • Also, a bit of dirt is unlikely to contaminate the pads (think of the conditions MTB's are often ridden in).
    – Andy P
    Aug 14, 2018 at 11:36
  • But (@AndyP) grease from dirty hands could well be an issue.
    – Chris H
    Aug 14, 2018 at 11:41
  • @AndyP it's a road bike (hybrid) rather than an MTB but yeah I'd expect a certain amount of dirt to get onto the rotor in normal use (and thus get transferred onto the pads). I can pull the brake lever all the way down, with not much resistance. Do you think I need to top up the oil/bleed the air in the brake system? It's weird that it would happen as a result of taking them out and putting them back.
    – Max Williams
    Aug 14, 2018 at 12:09
  • @MaxWilliams if you can pull the lever all the way with little resistance, you have probably managed to lose fluid out of the system somehow. Is it possible you removed the bleed port screw? To 'top up' the system you will need to do a bleed - plenty of guides and kits available online, or your local shop will do it for around £10
    – Andy P
    Aug 14, 2018 at 13:46

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If you set back the pistons (which is actually unnecessary if you are just inspecting the pads), pump the brake lever to move the pads back onto the rotor.

You can also clean the rotor with some solvent to make sure it is not contaminated with any grease or oil.

Update:

You can also check to see if you have a sticky piston in one side of the caliper that is not advancing properly. Unsticking them is easy, and guides on how to do so are easily found.

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  • Thanks - can you expand a bit on "pump the brake lever"? Do you mean repeatedly press it in as far as I can, and release it?
    – Max Williams
    Aug 14, 2018 at 12:10
  • @MaxWilliams Yes exactly that. Hydraulic brakes are self adjusting. As the pads wear a valve lets small amounts of fluid into the line between the lever and caliper each time you pull the lever under normal braking. If you set the pistons back you have to replicate that by repeatedly pulling the lever.
    – Argenti Apparatus
    Aug 14, 2018 at 12:37
  • I tried this at lunch time (pressing it in 6 or 7 times) and it didn't seem to make much difference - should i keep trying it?
    – Max Williams
    Aug 14, 2018 at 15:25
  • @MaxWilliams that should be sufficient. if there is proper resistance and pressure at the lever (does not go all way back to the bars) start looking at the caliper pads or rotor.
    – Argenti Apparatus
    Aug 14, 2018 at 16:39
  • It seems a lot better today, after being used a bit. I don't know if it was the use or just time but it's pretty much back to how it was.
    – Max Williams
    Aug 15, 2018 at 7:30

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