On a recent long ride in the Colorado mountains, the rear Shimano R785 hydraulic brake failed on the road bike I was riding as I was descending from 12,000ft to 9,000ft on a -5% paved road in traffic. Not what anyone wants to happen to them.
The rear brake lever felt mushy for a minute or two, then lost all power. Oil had leaked out of the caliper, but it wasn’t clear what caused the leak. Shimano retrieved the caliper, the attached hose line and the rotor to diagnose the situation, and determined that the right ceramic piston had cracked, allowing oil to leak out and causing the brake to lose function.
Read a full description of the particular ride here, with photos and a 3D elevation chart.
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I was riding a Specialized Tarmac Pro Disc test bike with Shimano 685 levers and R785 calipers with 140mm RT-99 Ice Tech rotors front and rear, with stock Ice Tech pads that have the cooling fins. This test bike came from Specialized via a shop, and had been used before. I had been riding the bike for approximately a month — including on many long descents — before the oil leak occurred.
The day of the incident, both brake levers felt normal for the first half of a 150mi ride, with both brakes functioning normally, as they did for the prior month of testing.
I could get the discs to squeal on really steep, really tight descents sometimes, but always with perfectly dependable performance. I had not crashed or dropped the bike, so there was no external damage to the rear caliper or brake line. I had not changed the brake pads, but I don’t know what work had or had not been done on the bike before I received it.
Unanswered questions
- What could cause a ceramic piston to crack?
- Could surface damage to a piston that doesn’t initially result in oil leak lead to a cracked piston through the heat and pressure of sustained braking?
- Could someone have damaged the piston by using a screwdriver or another metal object to retract the pistons during a brake-pad change?
- What does the cracked piston look like? Would you please return the caliper and/or send me pictures?
- How many other cracked pistons of this R785 or M785 caliper have you seen?
- How exactly is the piston on the XTR caliper different from the R785/M785 (XT) caliper? Why isn't this used on the R785?
- How are riders supposed to check the quality of their brakes’ pistons?
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