I recently read this pretty scary story on BikeRadar:
Apparently, one of their test riders was testing a Specialized Tarmac Pro with Shimano R785 hydraulic disc brakes on a long ride into Rocky Mountain National Park, with some major climbs and descents -- with elevations between 5,300 and 12,000 feet. On a descent down Trail Ridge (reportedly the highest continuous highway in the U.S.) the rider felt his rear brake get spongy for a few seconds -- then nothing. The brake lever bottomed out at the handlebar, and the rear brake was completely dead. His front brake was not affected, so luckily he was able to stop without further incident. A cursory roadside check found oil on the chainstay below the caliper.
The cause of the brake failure was that the brake fluid leaked out of the caliper. According to the article, Shimano collected the brake caliper, hydraulic lines, and the rotor to investigate the incident. Their finding was a cracked ceramic piston inside the caliper, which then let the brake fluid escape. Under such circumstances, each pull on the brake lever would just pump the fluid out of the caliper until it was gone.
Shimano concluded that it was unlikely that heat buildup under braking caused the piston to crack, as they didn't see any other signs on the pads or rotors to indicate excessive heat. But then, it still leaves the question unanswered why did it crack? One also has to wonder if this was an isolated incident, or have there been others?
Shimano issued a response to BikeRadar, which can be read in full HERE. I enjoyed the following excerpts, though:
"We are very sorry for the oil leak and trouble it caused on your ride." Yeah - "trouble." That's an understatement.
After explaining about the cracked piston (without being able to explain why), it concludes:
"Again, we are sorry for the trouble you experienced with this brake caliper. This is a rare occurrence for us, we will continue to study for further refinement and improvement.
"We always recommend that you inspect your braking system prior to riding. Disc brake system trouble may start to appear as a spongy feeling at the brake lever. A visual check of pad wear, contact of the pad to the rotor, and motion of the piston are all recommended. If you experience any problems we recommend you seek professional service at a bike dealer."
Of course, when the spongy feeling presents itself suddenly mid-descent (followed by total braking loss), doing the visual checks and seeking professional service at a bike dealer are kind of precluded by first being able to stop without getting killed.
Such an incident shouldn't necessarily be seen as a wholesale indictment against disc brakes on bicycles, but to my view it does highlight a certain problem with the brakes, as well as a lot of the new technology we're seeing on today's w nderbikes. On traditional cable-operated rim brakes, everything is pretty well out in the open, easy to see, easy to understand -- just as with most traditional bicycle components. On a hydraulic disc brake, a lot of the critical componentry is hidden inside. One can't visually inspect the caliper for a cracked piston the way they can spot a frayed cable. A faulty master cylinder can look no different than a good one. If there's something wrong, it can be harder to detect before it fails -- and if failure does occur, the results can be dire. Not only that, but diagnosing or repairing the problem can be much more difficult.
Somebody is bound to point out that we use hydraulic disc brakes in our cars without incident, and we put complete faith in them. But it's also worth noting that all the brake components in a car are much larger, and much more robust than those on a bike. In order to bring the weight down to a level that a cyclist can tolerate, the discs, the calipers, master cylinders -- everything -- has to be pared down to tiny scale, making them more prone (it seems to me) for failure.
Not that I needed another reason to stay happy with my traditional rim brakes, but something like this should still keep people alert to the potential downsides of the latest tech.