Forum home Road cycling forum Workshop

Sticky Caliper Piston - Shimano RS 505

photonic69photonic69 Posts: 1,242
edited October 2018 in Workshop
Dear forumites

I've a Felt bike with 105 Hydraulic brakes. The rear caliper seems to have a sticky outboard piston. After descending a hill and braking the outer pad still seems to rub the disc making a "tsss..tss..tsss.." sound with each rotation. This will last for a minute or so then after a while it goes until the next heavy application of brakes. The brakes have been centred many times but won't solve the issue. The "tsss..tsss...tss.." sound would indicate a run-out of the disc but this is so slight it is hardly noticable.

Any ideas. I hate sounds that shouldn't be there when riding.

Posts

  • sam_anonsam_anon Posts: 165
    You can remove your wheel and pads and pump the pistons out against a screwdriver or ruler to stop them popping out completely (which they might, so be careful).

    You can then clean outside of pistons and push them back in. Just be careful not to pop one out when you push the other in, if that makes sense.

    ...
  • keef66keef66 Posts: 13,123
    I found that with a brand new bike I was messing with for a colleague. I had never worked with disc brakes on a bike, but concluded the disc had to be very slightly bent.
    With great trepidation I carefully tweaked it with a massive adjustable wrench. Glorious tsk-free silence! 8)
  • Vino'sGhostVino'sGhost Posts: 4,129
    keef66 wrote:
    I found that with a brand new bike I was messing with for a colleague. I had never worked with disc brakes on a bike, but concluded the disc had to be very slightly bent.
    With great trepidation I carefully tweaked it with a massive adjustable wrench. Glorious tsk-free silence! 8)
    Ha was his trepidation greater than yours :)
  • Sounds more like the rotor warps slightly as it expands due to heat buildup from the braking, and as it cools it goes back to being straight. That would accord more with the symptoms you describe than a sticking piston.
  • keef66keef66 Posts: 13,123
    keef66 wrote:
    I found that with a brand new bike I was messing with for a colleague. I had never worked with disc brakes on a bike, but concluded the disc had to be very slightly bent.
    With great trepidation I carefully tweaked it with a massive adjustable wrench. Glorious tsk-free silence! 8)
    Ha was his trepidation greater than yours :)

    Fortunately I was in the UK and he was in Germany, so he was in blissful ignorance. The sphincter tightening was all mine!
    It was a long service award the UK company had to buy here and ship over. After the couriers took the thing on a 5 day tour of Europe but failed to deliver it, I had it stashed in the office here pending shipment to the Fatherland with his house contents. And since it was here I offered to spend a couple of lunchtimes fitting a few accessories for him. I never mentioned the disc bending thing...
  • plodder73plodder73 Posts: 318
    What Nick Payne said above, mine do that, they heat up, make a noise, cool down and go quiet. One of the joys of discs. I've learnt to live with it.
  • plodder73plodder73 Posts: 318
    Forgot to say I have RS505 as well. You can try finned pads and fancier rotors which help disperse heat a little better.
  • Vino'sGhostVino'sGhost Posts: 4,129
    keef66 wrote:
    keef66 wrote:
    I found that with a brand new bike I was messing with for a colleague. I had never worked with disc brakes on a bike, but concluded the disc had to be very slightly bent.
    With great trepidation I carefully tweaked it with a massive adjustable wrench. Glorious tsk-free silence! 8)
    Ha was his trepidation greater than yours :)

    Fortunately I was in the UK and he was in Germany, so he was in blissful ignorance. The sphincter tightening was all mine!
    It was a long service award the UK company had to buy here and ship over. After the couriers took the thing on a 5 day tour of Europe but failed to deliver it, I had it stashed in the office here pending shipment to the Fatherland with his house contents. And since it was here I offered to spend a couple of lunchtimes fitting a few accessories for him. I never mentioned the disc bending thing...


    nice long service award!
  • keef66keef66 Posts: 13,123
    nice long service award!

    Indeed! £1500 for 30 years. I bagged him a bargain Genesis Datum in the sales.

    I just made 20 years and to everyone's astonishment spent the £1000 on a cordless lawnmower and some Sonos speakers. I already had enough bikes...
  • Vino'sGhostVino'sGhost Posts: 4,129
    keef66 wrote:
    nice long service award!

    Indeed! £1500 for 30 years. I bagged him a bargain Genesis Datum in the sales.

    I just made 20 years and to everyone's astonishment spent the £1000 on a cordless lawnmower and some Sonos speakers. I already had enough bikes...

    thats properly generous . Result
  • photonic69photonic69 Posts: 1,242
    Enough about bonuses etc already. This is about sticky calliper pistons!!

    No, it's not the rotor heating and warping. It happens with even gentle braking though obvs for not as long

    Piston is not retracting as quickly as it should. Calliper and piston exterior have been cleaned

    What is the remedy?
  • keef66keef66 Posts: 13,123
    If the fault is with the piston it suggests either it’s damaged or gunked up so it’s not sliding freely in the bore, or the return spring isn’t doing it’s thing
  • I have had many a sticky piston in my lifetime and have solved every case by removing the pads, pushing the pistons out one at a time leaving enough to clean the side of the pistons (be careful not to push the piston out completely) followed by smearing the side of the pistons with Hunter's silicone lube on a cotton bud.
  • photonic69photonic69 Posts: 1,242
    The bike is new(ish) 2.5 months old. About 700 miles.
    Shop has cleaned and lubed the piston and the retaining bolt and "exercised" the piston, so in their eyes it is working OK. It is better than it was but still irks me. I'm imagining all those milliwatts of power being wasted due to the rubbing. I need every single watt of power to help me! More it is the sound that annoyes as it should not be doing this.
  • keef66keef66 Posts: 13,123
    I'm suddenly curious to get my hands on a hydraulic caliper and take it to bits.

    I've tinkered with hydraulic brakes on cars for years, both disc and drum, and marvelled at how simple they are. I'm guessing a bike is more or less the same, but smaller, and with the added refinement of a spring to retract the pads?

    Why the reluctance to eject the piston completely from it's bore? Are the seals so tight and the tolerances so close that they are hard to get back in??
  • photonic69photonic69 Posts: 1,242
    keef66 wrote:
    I'm suddenly curious to get my hands on a hydraulic caliper and take it to bits.

    I've tinkered with hydraulic brakes on cars for years, both disc and drum, and marvelled at how simple they are. I'm guessing a bike is more or less the same, but smaller, and with the added refinement of a spring to retract the pads?

    Why the reluctance to eject the piston completely from it's bore? Are the seals so tight and the tolerances so close that they are hard to get back in??

    The concept is basically the same. Most cars usually have one piston and the caliper slides. Bikes usually have two pistons (exotic Downhill MTB's can have four). The problem lies in the fact that rebuild kits with seals etc are virtually impossible to get hold of. To properly fix a brake caliper you'd need to split the two halves to get good access to the pistons and seals so you'd need a new gasket to fit the halves back together again. I suppose you could cut your own from a sheet but if it was a rubber ring gasket then you'd be stuffed.

    I too have rebuilt car brake calipers when nipples have snapped off and pistons have become sticky. It was easy - pop down to your motorfactors with the bits and get (order) replacements and shine up the piston with 2400grit wet and dry. Clean with Meths. Assemble. Bleed. Test. Away you go!
  • When I get a sticky piston I tend to wipe it on the quilt or pillowcase if it's a one night stand.
  • pilot_petepilot_pete Posts: 2,100
    Are you sure it is a sticky piston? Sounds more like you have a little bit of air in the system to me. As you brake the caliper heats up, the trapped air expands slightly forcing the piston(s) out and you get rotor rub. Once you have stopped braking the caliper cools down and the air contracts again thus the pistons move back to their ‘off’ position. I’ve had this before and it can be a bit of a pain to completely remove all the air on certain setups.

    I placed the bike in a workstand and propped the front end up on blocks of wood. This meant the hose to the rear caliper had a completely ‘uphill’ run to the lever reservoir. If you don’t prop the front end up higher than the caliper the hose remains horizontal from the bottom bracket area to the rear caliper, usually with a slight uphill section where it goes into the caliper. The principle is that any air bubbles in the brake fluid will rise to the top.

    Bled like this and with copious amounts of tapping the hose along its length to encourage any tiny bubbles to rise to the reservoir eventually did the trick. This is the first thing I would do rather than removing pistons etc.

    PP
Sign In or Register to comment.