Title says it all. Any ideas why?
What happened: went for a ride and maybe 3-4 miles into it i noticed the bike struggling a little. Over the next mile there were a lot of lights and, after one, I noticed the bike wouldn't roll forward in neutral even though I was downhill. Had to throttle it hard to get to the side of the rode. I was able to take whole caliper off, zip-tie the brake line to the bike (and tape up hole so it didn't leak), and return home safely.
Possibly relevant info, or not: brake fluid replaced/bled ~1 yr ago (3000 mi). Had some work done recently that required removal of rear wheel (and thus caliper as well), however, nothing that required actual work on the brake line/pads/calipers. I've ridden maybe 500 miles since then without symptoms. No visual problems on inspection.
I figure it is either something with the calipers or master cylinder, so I will inspect those this week. But I'm not knowledgeable to know what I should be looking for.
First idea what comes to my mind is a faulty master cylinder. Was the break pedal 'hard' when this happened?
Not sure what you mean by hard. For normal braking I use front brakes assisted with light rear brake use. In this case, the rear brake use did not feel any different (harder, softer, or anything else).
Really, everything about the bike felt, sounded, and ran normal and the first and only clue was that I was in second and it seemed like the bike was working harder than normal to go the speed I was going. As I hit the string of lights I would use the rear brake to stop and I think it was then that the brake was only working one-way, namely, it was compressing but not uncompressing when I released. So each consecutive light it seized more until my rear wheel was locked.
But yeah, nothing felt unusually "hard" until it was seized.
Any changes in rear sets or adjustments to the pedal?
I had that problem once. It was caused by the adjustment screw on the brake lever being screwed in too far. Basically the screw was causing the lever to always be just a little pressed, which obviously in turn made the caliper just a little pressed. It would run fine for a while but that added pressure would build up heat, expanding the brake fluid to the point of squeezing the rotor like I was on the brakes.
Quote from: ducpainter on April 27, 2014, 01:03:36 PM
Any changes in rear sets or adjustments to the pedal?
None. That said, I can't speak for the shop I had it at a few weeks ago. However, they were just replacing bolts on my rotor (two were starting to strip) and doing a some spot checks on my rear drive/wheel (I had just installed a new wheel & sprockets and asked that they check the work just to make sure I didn't grossly miss something), so I don't think they would have done anything with the brake other than move it aside.
Question for the crowd: I get how the hydraulics work to squeeze the pads on the rotor, but what mechanism/physics causes the pads to spread apart when the pedal is released?
Quote from: ChrisK on April 27, 2014, 01:22:55 PM
I had that problem once. It was caused by the adjustment screw on the brake lever being screwed in too far. Basically the screw was causing the lever to always be just a little pressed, which obviously in turn made the caliper just a little pressed. It would run fine for a while but that added pressure would build up heat, expanding the brake fluid to the point of squeezing the rotor like I was on the brakes.
Thanks. I'll check this when I head over to my garage later tonight (to clarify: I live in a city apartment, so "heading over to my garage" is actually a thing).
They don't really spread apart, it's just that there is no pressure on the pads so they just skim the rotor. The usual cause that DP was referring to is if the pedal were adjusted or moved it may not be returning enough to open the return port in the master, so every time you press the pedal pressure is applied but can't be released.
The seal on the piston is square cut and twists a little on application. When pressure is released the piston returns as the seal straightens. As Ddan said, there will still be some drag. Assuming the master and caliper are good the two most likely causes are either a misadjusted pedal or overfull brake reservoir so check them first. You want 1.5 to 2mm. free play at the lever. Going to Matchless tomorrow?
in the master cylinder bore there are two holes through to the fluid entry port or reservoir. the one closest to the outlet of the master cylinder is called the compensating port. this port is open when the piston is fully retracted (pedal at rest) and is closes as the piston moves into the bore and the seal goes past it.
its job is to allow fluid to flow back into the reservoir from the master cylinder bore and back the other way. as the brakes are used (even lightly) the fluid gets hot and expands. as the fluid expands a little flows back through the compensating port into the reservoir when the piston is fully retracted (pedal at rest) and so the expansion doesn't cause an increase in pressure in the brake hose downstream of the master cylinder.
if the compensating port is blocked and fluid can't flow back, you get a pressure rise in the system. this pressure rise tends to be an accelerating issue, as in the more pressure there is, the more the brakes drag, the more heat goes into the fluid and more the pressure increases.
the most common cause of a blocked compensating port is a piston not fully retracted. on european bikes (but not many japanese bikes it seems, i don't work on them), and on the brembo systems, the pedal rest position is fixed by the stop that activates the light switch. so the pedal has a fixed limit of retraction. between the pedal and piston is the pushrod, which is generally a two piece part where one threads inside another. if this is too long, the fact that the pedal end is fixed means that the piston has to compensate and will not retract fully out of the master cylinder bore. if it doesn't retract enough to open the compensating port to allow the fluid in front of the piston to flow back into the reservior as required, you're in trouble.
generally causes of this are owners adjusting their freeplay without realising the consequences, or fitting rear sets and not paying attention to it, or pushrod adjuster locking nuts coming loose and allowing the adjuster to slowly wind itself out (seen that a few times).
another symptom is that it may not bleed, which is because you can push fluid out by pushing down the pedal and opening a bleeder. but when you release the pedal, no fluid can flow into the master cylinder bore in front of the piston, so you have a vacuum in the system. if the adjuster is wound out to far, winding it back in with the above condition can/will result in the fluid level dropping in the reservoir as the piston retracts and the compensating port opens. removing a lever will also allow the system to refill itself if there is a vacuum, and then the next ride will result in the brake jamming on when nothing has actually been "adjusted".
also if you've had a bike that hasn't had a brake fluid change or has sat for a long time, these ports can just block with crystalised brake fluid. i had a m600 that i didn't even make it around the (very small) block at my factory on before both ends had locked up. i pretty much dragged it back to the factory.
if you want to see how fast this can happen, take the clearance out of the front brakes. even with very little application it will stop the bike in a few minutes of riding. the front you notice quickly, the rear often is on fire before a rider knows about it.
you can test for this by cracking a bleeder and releasing some fluid. if the wheel unlocks, this is the issue.
i've seen lots of melted rear calipers over the years. the adjustment of the pushrod has almost always been the cause. i say almost because i can't actually think of an instance where it wasn't, but saying always tends to upset people on this topic. it can happen just after it has been touched, or with a bike that has not been touched for some time prior. it's hard to know with owners who are sometimes embarrassed by generating a failure like this and maybe not telling the truth, but it's also very easy to get caught on even if the adjustment is just a touch off.
Brad... [clap] [clap] [clap]
Quote from: brad black on April 27, 2014, 07:33:38 PM
... generally causes of this are owners adjusting their freeplay without realising the consequences...
That's exactly what happened to me a couple years ago when I had this problem, thanks for sharing all that Brad.
Agreed. Thanks a bunch, Brad. I will need to re-read once or twice to truly get all the nuances of what's going on. Then I will apply to my investigation. Thanks 11tybilion. [beer]
Quote from: howie on April 27, 2014, 05:17:18 PM
Going to Matchless tomorrow?
Yup. See you there.
If the brake fluid is no older than a year it is probably one of the adjustments. Your shop manual will specify the starting dimension for the pushrod adjustment Brad mentioned. This is an example of such and is for a M1100evo.
(http://i165.photobucket.com/albums/u76/oldndumb/M1100EVOrearbrakeadjustment.jpg) (http://s165.photobucket.com/user/oldndumb/media/M1100EVOrearbrakeadjustment.jpg.html)
there are two lock nuts you need to check. #25 from the above picture, and the one on the stop that activates the brake light switch. either one can come loose and allow their threaded rods to move and take out the free play.
Quote from: brad black on April 28, 2014, 03:49:56 PM
there are two lock nuts you need to check. #25 from the above picture, and the one on the stop that activates the brake light switch. either one can come loose and allow their threaded rods to move and take out the free play.
Great. Thanks again.