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Rear brake not disengaging

Discussion in 'Technical and Troubleshooting Torque' at netrider.net.au started by Suriag, Dec 25, 2014.

  1. 2011 Aprilia Mana GT ABS 850

    So to begin by humiliating myself for completeness' sake: A month ago I apparently mistook the sounds of tortured scraping metal on my newly purchased 2011 Aprilia Mana GT ABS 850 for the sounds of my new engine (it was an upgrade from a 250...) and merrily continued to ride it for week(s). This resulted in a disk that was visibly warped and backing plates that had apparently partly melted, and unsurprisingly required everything from caliper on down to be replaced. The mechanic asserts that I must have caused the problem in the first place by riding the rear brake far more than I believe(d) I had been... it had good pads when he saw it only a month before that.

    They finally got and installed the new caliper etc and I rode it home yesterday - straight up the Great Western Highway with virtually no cause to use my brakes and being paranoid about what i'd been told caused the problem.

    Today I try getting it out of the garage and need my whole bodyweight to move it. I can ~hear~ the grinding of the pad on the disk. I look at the disk and it is already more scratched and worn than the front and the pads are definitely gripping tightly and already visibly wearing slightly. It is possible that i'm now being overly paranoid and seeing symptoms where none exist, but my recollection of the bike when I first got it was that it was not hard to move on flat concrete... and i'm sure the rear disk shouldn't already look older than the front ones.

    Either I have done something monumentally retarded, or they have fixed the symptom and not looked for the problem because they (reasonably) assumed it was my fault. I have no idea to figure out which or how to ask the mechanic without potentially insulting them, and i'm hoping that was enough embarassing detail to get some insight. (or reassurance if i'm jumping at shadows)

    (Picture is from the disk/pads today after I put it back in the garage - not going to ride it while i'm worried about repeating the damage.)


    Attached Files:

  2. How much free travel ( ie. before it actually starts to have more than the return springs resistance ) in the rear brake pedal ?
    More travel than normal may/would/could indicate a seized or jammed master cylinder.

    Having undone the caliper retaining bolts, working the caliper side to side while still over the disc....do the pistons retract and the pads free up ?
    If they don't, it may indicate a seized piston.

    There really are only a few possibilities here, seized master cylinder or seized caliper piston(s)
    Outside chance.....overfilling the brake fluid ( but it would be full to the brim to be at the point of causing the system to lock up )
  3. Thank you. I appreciate those leads and none of those options are ones i'll feel silly for keeping me off the Mana until it's sorted (or speaking to the dealer about after they re-open).

    It's hard to imagine an Aprilia mechanic making that dramatic mistake with the brake fluid.

    I'll have a look at undoing the bolts in a day or two when I can borrow some tools.

    There is buggerall free travel, afaic. Though since the invoice says they replaced the Caliper but not the master cylinder I wonder if i'm checking that properly - now I have some more specific keywords i'll check some videos of that.
  4. Its hard to tell from the pic, but it looks like only one side of the caliper has a piston. If that is the case then there is another potential cause, that is the slide pins are sticking, although if the caliper is new I would think fairly unlikely.

    Or, as perhaps the free travel on the footlever is non-existant holding the master cylinder partly depressed.

    A quick check to see if the master is holding the brakes on is to push with your hand quite hard on the outside of the caliper (assuming single piston caliper) and see whether you can get the piston to retract into the caliper at all. If you cannot, then there is something preventing the fluid from returning to the reservoir, ie master cylinder jammed or somehow held partly into it's stroke. There must be free play between the lever and the piston rod for proper operation.

    My advice, check everything very carefully from the lever back. Free travel, the master cylinder operation, and the caliper for correct movement on the pins as well as piston action

    Good luck.
  5. An oldy but a goody is if they used Dot 5 brake fluid,a big no no in Brembro brakes.Its Silicon based and makes the seals swell.It should take Dot 4 or Dot5.1.Well know in Italian bike secene.Make sure the pedal is not to high and engaging with the weight of your foot as well.Other than that what did he think you did,there there to be used.You payed to have it fixed and there is still an issue,be polite but make you point,finger pointing by the mechanic is a bit of a dick move.
    • Agree Agree x 1
  6. I had a bike once that needed a spring in the foot brake lever, in order to stop the weight of it applying the rear brake. It rubbed just enough to generate heat which caused everything to expand, which caused the rear brake to lock on.

    Replace the spring and all was good.

    Check if your mechanism isn't missing something like this spring.

    Or your just ridding with your foot on the lever a little.
  7. Seized or excessively sticky slave cyl, vs something "further up" I had a similar problem a while back on an opposing twin piston setup.

    I first bled just a little fluid to confirm that there wasn't a build up of pressure, and that the master cylinder was operating as it should. I got no release of pressure upon opening the bleed valve and the cylinder pumped brake fluid out as expected in a normally operating brake system. (easy test to localise the problem)

    I then removed the calliper and inserted a thin piece of wood between the pads and tried the brake. One piston moved, the opposing one didn't. - stuck piston. I could push the moving piston back, but couldn't budge the other one. Obtained a calliper seal kit and one spare piston just to make sure. I didn't end up using the piston, as it was OK, and the original was not damaged in removal, but the seals and dust seals were deteriorating - the cause of the sticking. All replaced, (I took the calliper to a shop and had them do it. It is sometimes very hard to remove stuck pistons. I have no specialised tools) It was fine. Refitted and bled the brakes with new fluid myself.
  8. I am with Zim, before you start working on it yourself and possibly voiding your warranty take it back to the dealer. Politely speak to the Service manager, and say they have not resolved the problem can they look at it again.
  9. I probably wasn't giving them enough credit anyway, they've been fantastic to me so far. Still, that's launched me off on some interesting learning about the brakes~

    TY all.
  10. To test if the brake is dragging: Get ready for a short ride, say 5-10 minutes. If you have a thermometer check the temperature of both front & rear disks. Start it up, pump the rear a few times before moving off then keep your foot well away from the brake pedal for the entire ride. Hang your foot out to the side or something. When you get back, check the the temperature of the rear disk if it's dragging it'll have warmed up appreciably. DANGER! I have been desperate (stupid) with a car rear disk & used a finger. "F#%c that's 'n hot. Shit, shit, shit. Time for some burn cream".
  11. I'm with the others in that the dealer needs to fix this. They have mis-diagnosed the problem and need to made aware of this. For future reference, you can release the bleed nipple to relieve the pressure if your master is seized, or the very very very small fluid return hole is blocked. A symptom of this is no pedal/lever travel.