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


#1

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?


#2

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.


#3

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:


#4

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.


#5

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


#6

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.


#7

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.


#8

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


#9

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


#10

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.