BMW K bikes (Bricks)

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1Back to top Go down    Phantom Back Brake on Wed Mar 23, 2011 9:46 am

BigWillyC


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Hi all i have a problem with the back brake on my K100RT - sometimes it works and sometimes it doesn't. It worked for the MOT a couple of weeks ago despite not working the day before but hasn't worked since...? Any ideas?

    

2Back to top Go down    Re: Phantom Back Brake on Wed Mar 23, 2011 3:23 pm

Guest


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D'you mean it has no pressure and doesn't apply the pads to the disc? That could be that it just needs a good bleed to remove air bubbles and such, or that the caliper piston is hanging up and it needs a rebuild. Maybe the pins are corroded and just need a cleaning. How's the fluid level and how thick are the linings? Pop the plastic caliper cover off with the end of a flat screwdriver and have a squiz. There should be a couple of mm's on each side. The fact that it worked and then didn't suggests the piston is sticking in the bore. With the cover off apply the foot brake firmly with your hand to see if you can spot the pads moving towards and away from the disc as you press then release the lever. Let us know the result of your basic eyeball queries. That'll give us sumpin' more to go on.


Number 9 in this catalogue image.

    

3Back to top Go down    Re: Phantom Back Brake on Wed Mar 23, 2011 8:52 pm

RicK G

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The caliper is where I would look first. The piston could be hanging up on one side which makes the other just push on one side of the disc instead of grabing it from both sides. Air would be unlikely as it would probably do it all the time and same for master cylinder but don't rule out any of them.

    

4Back to top Go down    Re: Phantom Back Brake on Wed Mar 23, 2011 9:28 pm

japuentes

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Hi there, I had all the problems: noising, spongy, piston stiking in the bore, pulsing, master cilynder leaks. Checking I found: crude, brittle seals and dust covers, uneven wore pads, wore rear rotor, normal in a 23 years system (and still braking when you really need it).The solution was new rear rotor (EBC), pads (EBC), repair kits for calipers and master cilinders and a good and carefull cleaning and system flush, during the overhaul and now the bike is braking like new.
One single job, perhaps not cheap, but I dont have to worry.
Best regards
JAP

    

5Back to top Go down    Re: Phantom Back Brake on Wed Mar 23, 2011 9:45 pm

Guest


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My back caliper was hanging up a bit and wearing the pads too quickly, dusting the back wheel up good. I took the caliper off the bike, pressed an air nozzle to the hole where the bleed screw goes and gave it a quick blast of air, popping the stuck piston out of the bore quick smart. I gave it a thorough cleaning, lightly sanding the scores in the bore with fine fine emery as well as the piston itself, then I whacked it all back together with this seal kit (BMW part number 34 11 2 310 493, about $36.00 in the US), bled the line and fitted new pads (Acer from the boys at the 'Bins). That was a year and a half ago. She's still golden and I'm still laughing.

    

6Back to top Go down    Phantom Back Brake on Thu Mar 24, 2011 6:20 am

Dennis

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'Like your style here TWB, I've got a mate with a CBX who gets anal about calliper/piston overhaul ! Bloody annoying, as I tell him there's nothing that can go wrong, it's only a hole with a piston and a seal that doesn't hardly move !! You're right, just blow it out, a quick clean and get it back together. Make sure she's safe and up the road you go, how hard can it be ?

    

7Back to top Go down    Re: Phantom Back Brake on Thu Mar 24, 2011 6:24 am

BigWillyC


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Thanks fellas it's my day off today so i plan to do some of the investigating you mention. What is strange about it is that when it works - it works splendidly. When it doesn't, it literally doesn't work at all. I can pump the lever with my foot and there is no pressure in the system - so definately not an air issue. I think if grit had managed to get in and push the seals out i would more than likely still feel some pressure on the pedal. A few days later it will work fantastically again. When i bought it the guy was quick to point out to me that the rear pads were more or less brand new which suggests he had had problems too. Like i say i'll have a look today and report back...

    

8Back to top Go down    Phantom Back Brake on Thu Mar 24, 2011 6:36 am

Dennis

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Try this -
1. Remove the cap from the fluid reservoir and lift up the rubber to break the seal, then see what happens. Do this when it plays up.
2. Dismantle the master sylinder and check the cup seal, it could have a minute nick in it. I had this years ago on a Trials bike. The seal had been damaged during assembly sa it passed over a hole in the bore of the cylinder.

    

9Back to top Go down    Re: Phantom Back Brake on Thu Mar 24, 2011 1:33 pm

BigWillyC


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Ok so i ended up taking the caliper off after having exhausted all other options. One brake pad is worn right down almost do the plate whereas the other still has a lot of life left... I expected to see that one piston was seized but both pistons slid into their bores with minimal pressure from my fingertips and the seals look tip top. I didn't have time to hook it back up and pump them out but with the way they slid back in i can't see one being seized. Seems strange that one pad has worn down completely and the other very little tho...

    

10Back to top Go down    Re: Phantom Back Brake on Thu Mar 24, 2011 1:43 pm

japuentes

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Hi there, the pass bores are small and crude from seals wore etc. can block them. I found the hard way that is easier to overhaul the wole system.
Best regards
JAP

    

11Back to top Go down    Re: Phantom Back Brake on Thu Mar 24, 2011 9:51 pm

Guest


Guest
I'd whacked togther a naked R1100RS from three other wrecked bikes...first ride out I noticed the back brakes were a bit soft...thought to give 'em a bleed when I got back...then rocketed down the freeway to a mate's house, slowly deccelerating as I went. Funny, what's that, thought I. It was when I glanced back and down that I realised the rear disc was bright red and glowing. I pulled into his driveway just as the Paralever's rubber boot caught fire. A quick shot of a fire extinguisher put the mess out and I noticed I'd not correctly adjusted the freeplay from the rear master cylinder to the foot brake lever. As things heated up they expanded to eventually apply the brake just enough to make the brake disc look like one on a GP bike ridden by Ben Spies at the end of the straight as he dived into turn one. Now I'm more careful to set things up the right way before venturing out.

    

12Back to top Go down    Re: Phantom Back Brake on Fri Mar 25, 2011 1:02 pm

BigWillyC


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Hooked it back up again today and pumped one cylinder out just over half way - the other came out about a quarter and stopped. I clamped the half way one and pumped the other out before dismantling the caliper and cleaning it up. Fitted new brake pads and stuck it back on - pleased to say it's all up and running and i can't believe how straight forward it's been compared to some of the jap calipers i've worked on in the past! The cylinders were really clean - are they ceramic? The best thing i did was taking off the back wheel which made it a lot less fiddly to get the job done. Thanks for all the advice!

    

13Back to top Go down    Re: Phantom Back Brake on Fri Mar 25, 2011 8:41 pm

RicK G

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The pistons are aluminium with a hard anodizing and they are very wear resistant but like a ceramic very prone to damage if they are dropped or held with pliers.

    

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