Help me figure out why my disc brake won't stop me!

I’ve been having a frustrating time with my rear disc brake (Shimano BR-R785) on my CX bike lately. Running fine for a couple of years but really starting to have issues with it this year. Basically when I put in a fresh set of pads (resin or sintered) it’ll work great for maybe 3 weeks before fading to almost nothing (strangely they actually work better in the wet when they get into this state). I’ve tried bleeding the line, cleaned rotors with isopropyl alcohol, I thought the pads might be a bit glazed so sanded them back, I even changed the caliper a while back. No real improvements. Also seems odd that the front is and always has been working perfectly.

Anyone else experience similar issues or have a fix, something else I can try?

Could the pistons in the calliper of been activated whilst the pads/disc were not present? That can cause issues. Sounds like you know what you’re doing though so I doubt you pulled the break lever.

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This sometimes happens with disc brakes. Mostly on “Commuter bikes” which are used in all sorts of temperatures and weather, where the seals in the caliper leaks a tiny bit of brake fluid.
The remedy, is to clean the pads and rotors thoroghly (As you have done), and perform a new brake-in procedure of the pads. 10-15 brakedowns form 25 km/h to 0,5 km/h should fix the business. This will make the brakes work fine for a while. If you in fact have got a leak, the pads will keep getting contaminated. On my commuter bike this is a problem if I havent used it for more than 5 days. I “burn” of some of the contamination with braking, but after 5 days it is to little to late, and I have to do the above procedure.

The only real remedy is changing seals (might not be worth it), changing calipers or brakes. As my commute is on salted winter roads I know the seals will again be bad, so I keep fixing the issue intermittently :slight_smile:


Try sanding you brakes a bit, just a light scuff to get the upper layer of material off.
I have never heard of bike disks going glassy but motorcycle brakes and car brakes do.
Racing CX the rear brake can get a bit hot witch causes this glas like top layer on your brakes.
Try it, if it doesn’t work you at least have nice looking brakes rotors again.

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I really hope this isn’t the issue, I changed calipers a couple of months ago :sob:

I don’t think it’s that, it feels to me almost like the pads are contaminated but I don’t know how, I’m sure I haven’t touched them or anything.

Is brake lever working ok? It is your master cylinder that does all the job. Otherwise it does sound like contamination. Brake fluid is hard to get rid of. A tiny amount in the disc vent will spread across the disc reducing the efficiency a good bit.
Maybe just me, but after trying a number of disc cleaning sprays, none of them beat a simple dish washing liquid (like Fairy, etc) on a tissue. It bites extremely well and never squeaks.

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Is the rotor worn out. Sounds like you use it in foul conditions.

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I would flush the brake fluid…I had a similar issue with some XT brakes on my MTB…dirty fluid.

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Measures 1.70mm on the vernier so think I’m ok.

When I get a chance I think I’m just going to hit it with all the suggestions above again to see if that sorts it out. Clean rotor, sand pads and fresh bleed. Fingers crossed.