Evening - 1st post so apologies for errors or obvious questions..
Got a sudden screeching, juddering and loss of braking power whilst braking on a moderate hill - Ultegra hydraulic discs on a 2020 Trek Emonda.
Inspection at home showed discoloured pads and maybe a little oily fluid on the trailing edge of the left pad backing plate. Assumed there was contamination from something - there was a trace of fluid around the back of the caliper - near the bleed port.
I assumed I’d made some error when doing a routine bleed 200 miles previously - perhaps not tightened things up adequately. I made sure everything was closed properly, gave it all a clean and then I placed a set of old but just about OK pads in there. Noise and braking improved and then returned to normal once the rotor was properly cleaned too.
So I’ve fixed the problem but not really diagnosed it. I’ve been worried about a leaky caliper. I previously replaced the pads and did the bleed 200 miles earlier - great performance and no problems until this ride so I thought I’d done a competent job.
I’ve ridden the bike gently and then more quickly with increasing amounts of harder braking. I’ve tried to test the caliper as much as possible for leaks but it’s dry as a bone.
Could I really have been unlucky enough to pick something up from the road? Surely a leaky caliper would continue to leak? LBS says just ride and see what happens.
I had something very similar happen with an Ultegra caliper – noise and poor braking. Noticed a little fluid around the bleed port, so loosened and retightened that and cleaned everything up. No problems since.
Clean rotors usually look pretty shiny, with radial micro scratchlike textures around the braking surface. The textures very closely resemble what you’d see on a vinyl record, especially as the pads and rotors get worn in. Some rotors do look a little different, especially if ridden through mud, etc, but never too far different, in my experience.
You’d know if something was off if the braking surface takes on a more rainbow or darker hue, which could point to either contamination or glazing. For the former, I’d check the rear of the pads, which gets contacted by the protruding piston, for an oily ring. This could point to worn seals, cracks in the ceramic Ultegra pistons, etc. For the latter, the solution usually comes down to how you’re braking, or considering finned pads, like those from Shimano, Swisstop and AbsoluteBlack.
I’m usually on the fence with re-using old pads, as it’s hard to know their history, or where they’d been stored prior to use, which could lead to sub-par braking. But if you’re confident they’ve not been contaminated, I’ve had good luck with rinsing the pads prior to use, with dishwashing detergent.
FWIW, Shimano recommends pad replacement when the pad itself (excluding the backing plate) is less than 0.5mm, while SRAM recommends a minimum of 3mm (including backing plate), before replacement.
I think it has to have been contamination given the sudden issue and its disappearance with different pads and a cleaned rotor. Hopefully it will have been a one-off for some reason and not a leaking calliper but I”ll keep an eye for now and look out for the signs that you mention.
For info, both the ‘damaged’ and the ‘old’ pads were Shimano finned resin ones.
It’s worth checking the backs of the pads occasionally for obvious fluid “rings” (like drink rings on a coffee table). There may be some leaking coming through the piston itself.
The Shimano pistons are ceramic and have been known to crack, usually caused by overzealous seating of the pistons when replacing pads or bleeding. This is why it’s common for people to open up the bleed fill port at the lever when pushing the pads back. If the system has been filled when the pistons are not fully seated, then when you try to push the pistons back, there’s no “give” in the system and it presents undue stress on the piston itself, sometimes leading to cracking. It’s also a good idea to try to push them back straight and even to avoid damage.
