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TECHNICAL MOTOBRICK WRENCHING In Remembrance of Inge K. => The Motobrick Workshop => Topic started by: awfers on March 12, 2012, 08:31:35 AM
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Hi all,
I'm looking for a guide to rebuilding the front calipers on the K75. Factory manual mentions stuff like re-tapping the holes for the four bolts to remove the Loctite, pushing the pitons out with compressed air (which I don't have) as well as reseating the pistons with a special BMW tool.
Are there any other guides available for this that don't require these tools?
Thanks,
Awfers
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I've worked on several of these and many non-K brakes as well. It can be done w/o splitting the caliper.
Compressed air is very handy for removing the pistons from any caliper. You might, depending on whether they're frozen or not, be able to pull the pistons out with a pair of channel locks but you'll risk damaging the pistons. Remove the dust cover first. Still, if the piston(s) are not frozen, that could work. If you're planning on replacing the pistons--not a bad idea--it probably doesn't matter.
You don't need any special tool to re-insert the pistons. Put some [clean] brake fluid on them and push them in--strait. On these calipers you should be able to push them in by hand but on larger calipers I've used a C-clamp to re-seat them.
Tom
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Using compressed air to remove caliper pistons is not a good idea. PERIOD. If the piston is really stuck when it finally does let go it will come out of the bore like a bullet. Good way to get hurt. You can make up an adapter with a grease zert on it and use a grease gun to push the piston out. You can get more pressure with it and grease is non-compressible, so when the piston does come loose it will not fly across the room like a bullet. If making an adapter is not in your cards then go to an automotive brake shop and ask them to just remove the piston for you. A lever type grease gun is capable of 5-10,000 PSI. A whole lot more than you will get from an air compressor.
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Using compressed air to remove caliper pistons is not a good idea. PERIOD. If the piston is really stuck when it finally does let go it will come out of the bore like a bullet. Good way to get hurt. You can make up an adapter with a grease zert on it and use a grease gun to push the piston out. You can get more pressure with it and grease is non-compressible, so when the piston does come loose it will not fly across the room like a bullet. If making an adapter is not in your cards then go to an automotive brake shop and ask them to just remove the piston for you. A lever type grease gun is capable of 5-10,000 PSI. A whole lot more than you will get from an air compressor.
All that may be true and I don't recommend you put your face in front of the piston when using compressed air to remove it. But having worked in automotive shops when I was younger, they pretty much all use compressed air. I can still hear that familiar, "pop". Just the amount of time it would take to thread a grease gun into a caliper (and unthread it when done) makes it almost a non-starter in a busy auto shop. And that does not take into account the amount of time it would take to clean all the grease off of the caliper(s). Those guys (and gals) are under incredible pressure to get work in and out as fast as they can so the fastest method is the one that gets used. I still use compressed air and I've never had a problem, but then I don't put my face in front of the piston either. Fingers, maybe, face, no. YMMV.
Tom
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All that may be true and I don't recommend you put your face in front of the piston when using compressed air to remove it. But having worked in automotive shops when I was younger, they pretty much all use compressed air. I can still hear that familiar, "pop". Just the amount of time it would take to thread a grease gun into a caliper (and unthread it when done) makes it almost a non-starter in a busy auto shop. And that does not take into account the amount of time it would take to clean all the grease off of the caliper(s). Those guys (and gals) are under incredible pressure to get work in and out as fast as they can so the fastest method is the one that gets used. I still use compressed air and I've never had a problem, but then I don't put my face in front of the piston either. Fingers, maybe, face, no. YMMV.
Tom
I second the air removal method, just be sure to aim out of harms way.
Coworker shot a brake piston through a window one time.
Worlds best shop accident was when a tractor trailer tire blew up during inflation. Holy shit, akin to a bomb going off. Real rush, mandatory safety cage rule after the incident. Fortunately, NO one was injured.
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All that may be true and I don't recommend you put your face in front of the piston when using compressed air to remove it. But having worked in automotive shops when I was younger, they pretty much all use compressed air. I can still hear that familiar, "pop". Just the amount of time it would take to thread a grease gun into a caliper (and unthread it when done) makes it almost a non-starter in a busy auto shop. And that does not take into account the amount of time it would take to clean all the grease off of the caliper(s). Those guys (and gals) are under incredible pressure to get work in and out as fast as they can so the fastest method is the one that gets used. I still use compressed air and I've never had a problem, but then I don't put my face in front of the piston either. Fingers, maybe, face, no. YMMV.
Tom
I second the air removal method, just be sure to aim out of harms way.
Coworker shot a brake piston through a window one time.
Worlds best shop accident was when a tractor trailer tire blew up during inflation. Holy shit, akin to a bomb going off. Real rush, mandatory safety cage rule after the incident. Fortunately, NO one was injured.
For me it was a battery blowing up. Prolly not as big a bang as a trailer tire, but when they're dead, they're deadly. Acid and bits of lead plate everywhere.
Tom
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For me it was a battery blowing up. Prolly not as big a bang as a trailer tire, but when they're dead, they're deadly. Acid and bits of lead plate everywhere.
Tom
Ha, a battery exploding is also a real rush, BTDT
GoldWing falling off a lift was another. We ended up calling 911,paramedics, whole nine yards. Tech was struck in the head and rambling,best to have a medical professional check him out for insurance liability. Mental note to always tie down a bike when it's on a stand,NO exceptions.
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RE: compressed air and the danger involved.
I haven't done it to my BMW yet but when I re-built my Honda's calipers I was taught to put a common paint stirring stick in between when using the air method. The stick is thin enough to allow the pistons to move enough for removal but stops the pistons from flying across the room. I would think that method would work on any brand brake.?
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Thanks everyone.. guess I've got to get a compressor then...
Off to hornbach on Saturday...
Cheers!
Awfers
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Here's a web page that I stumbled across that you might find useful: Rebuilding the K75 front calipers (http://enderw88.wordpress.com/2008/12/09/rebuilding-the-k75-front-calipers/)
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Thanks everyone.. guess I've got to get a compressor then...
Off to hornbach on Saturday...
Cheers!
Awfers
Or you could try CubPilot's suggestion and try to take it to an automotive shop to have the piston removed.
That might be cheaper! :yow
Tom
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I had a case of rear brake only pushing on one brake pad so I rebuilt the bastard, but can't find a torque setting for the two bolts that hold it together. I know it's gotta be tight, and tight's tight, but anyone got that number to hand? I've got it back on the bike now so should be easier to reef on it...
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. . . can't find a torque setting for the two bolts that hold it together. . . .anyone got that number to hand? .
I use the same torque setting I use for the mounting bolts because the caliper halve bolts are the same type and diameter, just shorter. ~24 ft-bs ~34 Nm I tighten them in a couple of stages. Values are from Clymer's general torque table and brake-specific torque table.
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Here's a simpler way of removing the pistons. Pull the calliper away from the rotor, remove the brake pads, now place a bucket under the calliper fill the brake reservoir with old fluid (discard later) and pull the brake lever slowly. The brake pressure will push the pistons out without the energy and ballistics when using compressed air. Now remove the calliper from the brake line and rebuild.
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Thanks, Laitch! I’ll check that table. Think I saw you post it in another thread.
Next time, imiklosy!