Author Topic: Rear Disc brake half locked on  (Read 381 times)

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rob-linton

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on: December 21, 2020, 12:43:51 am
Hey all. First post here. I own a classic 500 Pegasus with ABS. A couple of years ago I ripped the front ABS sensor cable off the front wheel, no dramas, I’ve never replaced it and I ride it with the ABS disabled and have since then. However I’ve just replaced the OEM chain after riding it from Melbourne to Darwin here in Australia and I’ve noticed an issue that has started occurring with the rear brake. The brake shoes seem to become stuck and don’t disengage the entire way, leaving them rubbing on the disc constantly. The disc becomes red hot even when I don’t use the rear brake. I’ve bled the brake line and loosened the rear axle bolt and re-tightened it and all was good for about 10kms. Then the problem reoccurred.

It seems to be the the rear callipers don’t disengage the whole way.

No idea why, and google doesn’t seem to have the answer...so here I am. Happy to answer any questions or take photos if you need them.

Thanks in advance.


Ove

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Reply #1 on: December 21, 2020, 10:10:58 am
Check the fluid is properly bled, with no air in the system, check there are no hydraulic fluid leaks. When it happens, is the brake pedal fully returning? The spring on the pedal might be stretched, the pedal spindle might be sticking and need cleaning and lubricating. Strip down, clean and lube the spindle. Or the pistons in the calliper might be sticking due to dirt and dust, or corrosion. Strip and clean with a brake cleaner aerosol.


Haggis

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Reply #2 on: December 21, 2020, 11:38:13 am
The sliding pins are probably seized.  Strip and clean the calliper.
Use a silicone or rubber grease when rebuilding.
Off route, recalculate?


gizzo

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Reply #3 on: December 21, 2020, 11:02:32 pm
Hi Rob. Welcome to the forum. And big ups for riding an enfield from bottom to top. That's a big ride.

What they said plus my own input.

My 535 continental gt has just been through the exact same thing. I think there were a number of causes. Idk whether your Pegasus uses the same Bybre (second string Brembo) as my GT but here goes.

The sliding pins the caliper body slides on were bone dry. They ought to be lubricated. They're also a sloppy fit in the holes on the other side of the caliper. This lets the caliper cock over a bit when the brake's applied. My pads were worn into a wedge shape and it wouldnt surprise me if that wedge caused some kind of light servo action that lets the disc drag the pad onto it. That'd account for some of the drag.

After I put in new pads and greased the pins it was a lot better but the brake still dragged ever so slightly and would be hot and a tiny bit draggy after a ride even if I didn't touch the rear brake.

Next I pumped the piston all the way out, cleaned the crud from the outside (it'd been fully extended so the max length that could get filthy, was), new brake fluid and a bleed. It's been perfect since. While I was at it I noticed the reservoir lid was wet so I guess the new pads displaced enough fluid to overfill the pot. That may or may not have helped cause the problem.

Key points:
Clean and lubricate sliding pins

Remove and clean piston. Change seal if you need.

New brake pads and fluid.

Hope that helps. Your name sounds familiar. Do you write in one of the print mags or something?
simon from south Australia
Continental GT
Pantah
Monster
DR250
DRZ400SM


rob-linton

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Reply #4 on: March 24, 2021, 12:07:37 am
Thanks for all of your replies!

Gizzo, No I don't write in the print mags but the Royal Enfield trip up to Darwin was published in the AU Royal Enfield club magazine.

Here is the update and solution for everyone.

After bleeding the brakes and pulling apart the rear callipers and greasing the calliper pins with no solution in sight I changed my focus to the master cylinder.

After much head scratching I finally found the reason, which bizarrely I think is the same reason that some Royal Enfield's have been recalled for safety issues on the master brake cylinder.
(But not in AUS)

In the master brake cylinder there is a white sleeve used to guide the piston and hold in place the piston seal. This sleeve was not finished properly with the clearance being too small between the sleeve and the shaft of the piston causing it to grab on the return stroke.

The inside of the sleeve was rough and not finished so fine particles were able to lodge between the sleeve and the piston shaft, causing it to grab.

Polishing the inside of the sleeve with a piece of 2000 wet and dry wrapped around a drill bit solved the problem straight away.

The fact that I rode it through the middle of Australia and exposed it to a bunch of fine red dust probably didn't help matters.

Thanks for all your responses. Safe riding.


suitcasejefferson

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Reply #5 on: March 24, 2021, 05:19:45 am
Could be several things, but they are all fairly simple. There may still be pressure in the brake line, usually caused by a plugged return passage in the master cylinder. Or the piston could be sticking in the caliper bore. Or the pads could be sticking on the pins. Or they could be binding because something is out of alignment.

As for the ABS, IF I bought a bike with ABS, I would disable it before even riding it home from the dealer. I and I alone control MY motorcycle.
"I am a motorcyclist, NOT a biker"
"Some of the worst things imaginable have been done with the best of intentions'


Haggis

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Reply #6 on: March 24, 2021, 12:17:58 pm
 :)
« Last Edit: March 24, 2021, 12:22:50 pm by Haggis »
Off route, recalculate?