bout to make an ass of m'self...could my rear brake lock up...by itself?

[Rudemouthsky]

I posted in Crash analysis about a spill I had today. But the more I replay the event, the more I think the rear wheel locked up which was what caused me to go down. 3/4 finished with a slow left hand turn and since it happened so fast, I had no idea what happened. At first I thought I must've been hit, but I wasn't. Then I blamed my overloaded backpack...or thought I'd popped off the clutch too fast..or my balding tire..or maybe there was an oil slick. But I really feel like I remember the tire dragging, a violent drag on my rear end, then boom down.

I just replaced the rear caliper because of something similar 2 weeks ago. On the freeway I felt the bike bogging down hard. Immediately pulled over in case it was dying. No pressure in the brake pedal. Got home, took apart the caliper and found 2 cooked pistons. I figured I must've stepped on the brake accidentally even though that seemed really unlikely. When I replaced the caliper that was the hardest time I have ever had bleeding a brake line. I pumped, pumped, pumped, and even had to suck fluid through with my mouth at one point. And the pedal still seems mushy.

So, sorry for the long winded post, I guess I'm just wondering if my rear brake could be locking up by itself somehow. Twice, 2 different calipers.

[Ddan]

It's pretty easy to adjust all the freeplay out of the brake pedal and block off the fluid return port in the master.  That will lock up the rear brake

[ducpainter]

Do you have aftermarket rear sets?

I'll bet, if the rear is in fact locking, you have inadequate free play in the pedal.

Dan beat me to it...

[Rudemouthsky]

seems to be plenty of freeplay. Hrrm. What else? The caliper is new..could something be causing the fluid to overheat? The bike has 35,000 on the original master cylinder..kinda want to replace that now. Or just scrap the rear brake altogether. I'm afraid to ride it now :/

[ducpainter]

How are you measuring?

[Rudemouthsky]

measuring the freeplay? I'm not exactly...just judging the feel of it...

[ducpainter]

You need to be sure.

There should be 1.5 mm before the actuator rod hits the piston. Pull the rubber boot and look.

If there is that amount of free play and the caliper still locks up the piston in the master might not be returning all the way which would cause the same thing.

[Rudemouthsky]

Thanks DP I will check. 1.5 mm that is it? that's less than a fingernail...

[ducpainter]

Thanks DP I will check. 1.5 mm that is it? that's less than a fingernail...

It's .060, or 1/16"

[brad black]

ime any issue with the rear brakes dragging or locking it is always the freeplay being not, as it were.

i say remove the pushrod and make it very short.  then you'll know what freeplay feels like.  it means pushrod not touching piston.

you probably need to rebleed it all too.

[Slide Panda]

While you're working on stuff - ensure the guide pin on the caliper is clean. I had a pad hang on overly cruddy pin and cook my rear rotor.

[ducpainter]

While you're working on stuff - ensure the guide pin on the caliper is clean. I had a pad hang on overly cruddy pin and cook my rear rotor.

Good point, but he's already replaced the caliper.

[Slide Panda]

But was it a brandy new caliper? Didn't say 

[ducpainter]

But was it a brandy new caliper? Didn't say 

I'd imagine even if it was a used caliper the pads would need to be installed and a cruddy pin would be dealt with.

[zooom]

[ I'd imagine even if it was a used caliper the pads would need to be installed and a cruddy pin would be dealt with.

assumption can be the the mother of all f**kups....sometimes...LOL