grw
BMWNUT
A few weeks ago my box from Motobins showed up with brake rebuild parts. Just wanted to share my experience with the process.
I had a demonstrably warped left rotor. Both the front rotors were at the very end of the thickness spec so while they had some life left from a strict wear standpoint they were certainly not going to last that much longer. That ruled out machining the warped rotor.
The pulsing from the warped rotor was annoying but the real problem was that the brakes were sticking on slow stops making modulation very difficult. This was a safety issue and I had stopped riding the bike until I could sort it out.
Finally every since I had bought the bike in 2006 I had the distinct sense that the brakes were simply not working that well. I had ridden an '85 K100RT for six years (bought new in '85) and had a good sense of what a K100 brakes should feel like. The brakes on my current '85 required high lever effort even for slowing and had lousy feel. My first attempt to remedy this was installation of Spiegler stainless brake lines. This made a small difference but did not solve the problem.
Although I don't have much of a repair history for the bike I doubt the calipers had ever been apart on this '85 K100. I decided to replace the rotors and rebuild the calipers.
From Motobins I ordered a set of Grimeca rotors and two caliper rebuild kits along with stainless fasteners for the calipers, new pins and springs, etc.
The swap of the rotors went well. It's pretty simple. The new rotors mounted up fine and the new rotors plus the stainless bolts looked very nice. It's a good time to check the wheel bearings and they looked to be in good shape.
I had already removed the calipers (after draining the system) from the bike and when I split them and cleaned them they looked fine. I used compressed air to remove the pistons. It worked almost too well. You really have to aim into a towel or the pistons might go flying across the room. Don't ask me how I know.
The pistons were not scored or corroded. I decided to reuse the calipers and the pistons. New seals went into both calipers using the supplied seal lubricant and the calipers were reassembled using the new stainless bolts.
I should say how I cleaned the calipers and and pistons. This is going to make some people cringe. I drained as much fluid as I could and then soaked the pieces in a strong degreasing solution (water plus purple degreaser). I must have let them soak for 10 or 15 minutes. The water turned dark brown/black. After rinsing thoroughly under cold water I used compressed air to blow out the calipers halves and dry the the pistons. This cleaned the pieces remarkably well and I don't think the brief exposure to water harmed the components. After all they've been exposed to water in brake fluid for 25 years and they still look fine.
I installed new "European" pads from Motobins. These were less expensive than the stock BMW or standard after-market brand Motobins sells. They look like standard sintered pads with an almost identical backing plate to the stock pads. My understanding is that because pads bed to the rotors you should never use old pads with new rotors. I also used new pins and a new spring. The only old piece to go back on were the dust covers. They got cleaned up and were in fine shape. No reason to replace those.
The calipers went back on with little fuss. Again the new stainless mounting bolts looked good. I decided to order stainless brake line brackets next time around as the brackets (mounted on the fender bolts) were tired looking.
I had tried to replace the entire hand grip assembly including a new master cylinder and reservoir but had run into problems with the fit of the new version of the hand grip from BMW (fortunately purchased from my local dealer as it turns out; see related post). So I disassembled the the hand grip and master cylinder. I cleaned them the same way I did the calipers and reassembled. Afterwards I refilled the system with clean brake fluid and bled the air out.
My first ride was a revelation. The brakes were excellent, stopping the bike with a firm smooth squeeze of the lever. Slowing was accomplished by slight pressure instead of a full four fingered squeeze previously required. There was still some mushiness at the lever and I bled them again (mostly burping air out at the master cylinder). The brakes now feel good and are exactly the way I remember the original brakes on my old K100RT working. Properly used these brakes will stop the bike fast.
This past week I received the rear brake rebuild parts. The rear did not need a rotor right now so I just rebuilt the caliper. Note that the pictures and directions in the Clymer and Haynes manuals for the '85 K100 are not correct. They show a caliper which is identical to the front calipers. In fact the back caliper on my bike is slightly different (internally and externally). The rebuild kit comes with one o-ring and the caliper I had required two. The only thing I can think of is that because the line in and the bleed valve are on opposite sides of the caliper there are two passageways requiring o-rings to seal them instead of one as on the front calipers. In any case note that you may need two o-rings for a rear rebuild.
Again I used stainless hardware to re-assemble and remount the caliper. I put in a stainless bleeder valve, too. It's so much nicer looking than the rusty old parts.
I took the opportunity to pull the final drive and re-grease the splines. My final drive splines are not happy looking and a trip to Bruno's may be in the offing. New stainless bolts were installed for the final drive. I'm hoping to get through the summer (much finger crossing). I also de-gunked the clutch arm and cable nipples so I could see them well enough to adjust.
That's it. I'd say all together I had about 8 hours in the overhauls and inspections. This was my first time splitting calipers and the first time always takes longer. All I can say is that the improvement (rotors, pads, overhauled calipers) was worth it and I highly recommend this sort of maintenance if your older K-bike's brakes are not quite what they used to be.
I had a demonstrably warped left rotor. Both the front rotors were at the very end of the thickness spec so while they had some life left from a strict wear standpoint they were certainly not going to last that much longer. That ruled out machining the warped rotor.
The pulsing from the warped rotor was annoying but the real problem was that the brakes were sticking on slow stops making modulation very difficult. This was a safety issue and I had stopped riding the bike until I could sort it out.
Finally every since I had bought the bike in 2006 I had the distinct sense that the brakes were simply not working that well. I had ridden an '85 K100RT for six years (bought new in '85) and had a good sense of what a K100 brakes should feel like. The brakes on my current '85 required high lever effort even for slowing and had lousy feel. My first attempt to remedy this was installation of Spiegler stainless brake lines. This made a small difference but did not solve the problem.
Although I don't have much of a repair history for the bike I doubt the calipers had ever been apart on this '85 K100. I decided to replace the rotors and rebuild the calipers.
From Motobins I ordered a set of Grimeca rotors and two caliper rebuild kits along with stainless fasteners for the calipers, new pins and springs, etc.
The swap of the rotors went well. It's pretty simple. The new rotors mounted up fine and the new rotors plus the stainless bolts looked very nice. It's a good time to check the wheel bearings and they looked to be in good shape.
I had already removed the calipers (after draining the system) from the bike and when I split them and cleaned them they looked fine. I used compressed air to remove the pistons. It worked almost too well. You really have to aim into a towel or the pistons might go flying across the room. Don't ask me how I know.
The pistons were not scored or corroded. I decided to reuse the calipers and the pistons. New seals went into both calipers using the supplied seal lubricant and the calipers were reassembled using the new stainless bolts.
I should say how I cleaned the calipers and and pistons. This is going to make some people cringe. I drained as much fluid as I could and then soaked the pieces in a strong degreasing solution (water plus purple degreaser). I must have let them soak for 10 or 15 minutes. The water turned dark brown/black. After rinsing thoroughly under cold water I used compressed air to blow out the calipers halves and dry the the pistons. This cleaned the pieces remarkably well and I don't think the brief exposure to water harmed the components. After all they've been exposed to water in brake fluid for 25 years and they still look fine.
I installed new "European" pads from Motobins. These were less expensive than the stock BMW or standard after-market brand Motobins sells. They look like standard sintered pads with an almost identical backing plate to the stock pads. My understanding is that because pads bed to the rotors you should never use old pads with new rotors. I also used new pins and a new spring. The only old piece to go back on were the dust covers. They got cleaned up and were in fine shape. No reason to replace those.
The calipers went back on with little fuss. Again the new stainless mounting bolts looked good. I decided to order stainless brake line brackets next time around as the brackets (mounted on the fender bolts) were tired looking.
I had tried to replace the entire hand grip assembly including a new master cylinder and reservoir but had run into problems with the fit of the new version of the hand grip from BMW (fortunately purchased from my local dealer as it turns out; see related post). So I disassembled the the hand grip and master cylinder. I cleaned them the same way I did the calipers and reassembled. Afterwards I refilled the system with clean brake fluid and bled the air out.
My first ride was a revelation. The brakes were excellent, stopping the bike with a firm smooth squeeze of the lever. Slowing was accomplished by slight pressure instead of a full four fingered squeeze previously required. There was still some mushiness at the lever and I bled them again (mostly burping air out at the master cylinder). The brakes now feel good and are exactly the way I remember the original brakes on my old K100RT working. Properly used these brakes will stop the bike fast.
This past week I received the rear brake rebuild parts. The rear did not need a rotor right now so I just rebuilt the caliper. Note that the pictures and directions in the Clymer and Haynes manuals for the '85 K100 are not correct. They show a caliper which is identical to the front calipers. In fact the back caliper on my bike is slightly different (internally and externally). The rebuild kit comes with one o-ring and the caliper I had required two. The only thing I can think of is that because the line in and the bleed valve are on opposite sides of the caliper there are two passageways requiring o-rings to seal them instead of one as on the front calipers. In any case note that you may need two o-rings for a rear rebuild.
Again I used stainless hardware to re-assemble and remount the caliper. I put in a stainless bleeder valve, too. It's so much nicer looking than the rusty old parts.
I took the opportunity to pull the final drive and re-grease the splines. My final drive splines are not happy looking and a trip to Bruno's may be in the offing. New stainless bolts were installed for the final drive. I'm hoping to get through the summer (much finger crossing). I also de-gunked the clutch arm and cable nipples so I could see them well enough to adjust.
That's it. I'd say all together I had about 8 hours in the overhauls and inspections. This was my first time splitting calipers and the first time always takes longer. All I can say is that the improvement (rotors, pads, overhauled calipers) was worth it and I highly recommend this sort of maintenance if your older K-bike's brakes are not quite what they used to be.
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