BMW K bikes (Bricks)

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nullfox

nullfox
active member
active member
Last weekend I replaced my triple clamp with a new one from rebelmoto and painted my wheels. In the process of painting the wheels I had to remove the front wheel and had already been in the process of removing the handlebars so the front brake hose was not connected at the triple tree. I removed the front calipers and hung them by one of the bolt holes with a slack brake line as instructed.

I put the front wheel back on and remounted the calipers but my front brakes are frozen onto the disk and it takes force to move the front wheel. I thought maybe the pistons had come too far out so I took the calipers back off and I took a tape covered screwdriver in between the disks and twisted it to push the pistons all the way out. I also removed the cap from the MC, pumped the brakes for a bit and jiggled the bars to get all of the air out until the brake level had some feeling to it. I then remounted the calipers and closed the MC cap tight. There was a lot of daylight between the pad and the disk and as I progressively pumped the brake, it got closer and closer until the brakes were again frozen to the disk.

What the hell am I doing wrong here?

Also, I'm noticing that the front suspension doesn't want to rebound and seems a bit "stuck" and will often pop back as I've removing/remounting the brake calipers which wasn't happening before. Aside from a new triple clamp, I also removed the OEM fork bridge the fender mounted to and installed an after market plate for an aftermarket fender that says its for K bikes. Other than that, nothing has changed on the fork front and I'm confused as to why it's sticky now and I'd love any insights.


__________________________________________________
1988 K75C
    

Point-Seven-five

Point-Seven-five
Life time member
Life time member
For starters, when was the last time the bike was actually ridden?  Do you have any idea when the brakes were last flushed and the fluid replaced?  Are the brake lines the original rubber ones?

Your brake lines are 33 years old.  There are several possibilities of damage.  First is that the inner lining has collapsed and is not allowing the free flow of fluid to and from the caliper.  This is a fairly common problem and the main reason it's a very good idea to replace your brake lines with braided stainless steel whenever you get a bike with old lines.

the second possibility is that the lining is being shed in bits that are clogging up the passages in the lines fittings, and possibly the calipers as well.  I had that problem on one of my bikes.

Brake calipers, handlebars and fork rebound, oh my... Dscn3018
Brake calipers, handlebars and fork rebound, oh my... Dscn3017
Brake calipers, handlebars and fork rebound, oh my... Dscn3016

Remove the banjo fitting at the caliper and squeeze the brake lever a couple times.  Fluid should flow freely from the fitting.  Pump a good bit through and look for any mouse turds coming out of the line.  Maybe if this is your lucky day, the fluid will flow alright and the turds will flush out.  Still, I would replace the line.  A collapsed line will present itself as being very difficult to get fluid through, the more you pump the harder it will get.  You definitely have to replace it in that case.

As to the forks, I suspect that you didn't tighten the pinch bolts and fork brace hardware in the proper sequence.  It's very important to do it properly as very little misalignment will create all sorts of problems with the front end.


__________________________________________________
Present:
1994 K75RT
1991 K100RS


Past:
1988 K100RS SE
1994 BMW K75S
1992 BMW K100RS
1982 Honda FT500
1979 Honda XR185
1977 Honda XL125
1974 Honda XL125
1972 OSSA Pioneer 250
1968 Kawasaki 175
    

nullfox

nullfox
active member
active member
Thanks for the quick reply.

The bike was ridden last week from my apartment about 40 miles to my families ranch where I work on it - the brakes were working a-ok before and I've had the front wheel off previously and didn't have this issue after. The previous time all I had to do was put the bike on the side stand and pump the brake level to get air out and it worked immediately. This time, it hasn't worked. The brake line from the MC to the top of the brake line in the triple clamp is a new one from Venhill I installed about 8 months ago and has been working flawlessly.

I understand about the collapsed line but it seems to be presenting differently than that. It seems like the pistons retract less than they push out ultimately leading to being frozen on the disc. If I push the pistons/pads out manually I have tons of daylight between the disk and pad - as I pull the lever the pads push out towards the disk and when I let go, they retract, but it doesn't seem like all the way - so after about 5-6 pulls, the pads are stuck to the disk. I unfortunately don't have a break bleeder because I haven't had this issue before but maybe it's time to pick one up.

As for the front forks, that sounds reasonable - I've never removed the fork brace before. I have the manual but am struggling to find where it talks about the correct order for tightening bolts - do you happen to know what that order looks like?

Cheers


__________________________________________________
1988 K75C
    

Laitch

Laitch
Life time member
Life time member
Be certain these two ports in the master cylinder are not clogged and the piston itself is moving fully.
Brake calipers, handlebars and fork rebound, oh my... Master10


__________________________________________________
1995 K75 81,000 miles
Brake calipers, handlebars and fork rebound, oh my... Usa-lo10
    

Point-Seven-five

Point-Seven-five
Life time member
Life time member
Okay, it sounds like a daily rider with new lines, so no problem for not being used.

You don't need a special brake bleeder.  A gallon jug, 2 feet of 1/4" clear vinyl hose from the hardware store and a 7/16 or 11mm wrench is all you need.  I like to use an old windshield washer fluid jug because I can stiick the hose down into the handle so it doesn't pop out.  Fill the jug about 1/3 with water so it sits solid on the floor and after you pull the cap off the bleeder, put the closed end of your wrench on the bleeder and put the end of the hose on it.  

Now you're ready to go.  Do a good flush and bleed.  I'd pull the banjo fittings and run a bunch of fluid through them just to make sure there isn't any crap in the system.  

Before you bleed the brakes, get the front end put together.  Put the legs in the clamps and tighten only the top pinch bolts, the bottom pinch bolts should be barely finger tight.  Put on the fork brace and the front mudguard, again, all the fasteners are barely finger tight.  Put the axle and the wheel on, followed by the brake calipers, snug down the caliper bolts to a bit more than finger tight so the calipers can't move around.  Bottom pinch bolts are left loose.  The idea here is that everything is free to wiggle around.

Push the front wheel up against something solid and pump the front end up and down a couple times.  with everything loose, it should move up and down pretty easily.  Now torque the top and bottom triple clamp pinch bolts.  Bounce it again to make sure it moves easily then snug the bolts for the fork brace and mudguard.  Bounce it again, then jack up the front wheel so it's off the ground,  can you grab the axle and twist it in the bottom of the forks.  If so, tighten the screw at the end of the axle and then snug up all the lower pinch bolts.  Bounce it again and if it's moving like it should go around and torque all the hardware starting at the top and working down.  Now you can bleed the brakes.


__________________________________________________
Present:
1994 K75RT
1991 K100RS


Past:
1988 K100RS SE
1994 BMW K75S
1992 BMW K100RS
1982 Honda FT500
1979 Honda XR185
1977 Honda XL125
1974 Honda XL125
1972 OSSA Pioneer 250
1968 Kawasaki 175
    

nullfox

nullfox
active member
active member
Point-Seven-five wrote:Before you bleed the brakes, get the front end put together.  Put the legs in the clamps and tighten only the top pinch bolts, the bottom pinch bolts should be barely finger tight.  Put on the fork brace and the front mudguard, again, all the fasteners are barely finger tight.  Put the axle and the wheel on, followed by the brake calipers, snug down the caliper bolts to a bit more than finger tight so the calipers can't move around.  Bottom pinch bolts are left loose.  The idea here is that everything is free to wiggle around.

Push the front wheel up against something solid and pump the front end up and down a couple times.  with everything loose, it should move up and down pretty easily.  Now torque the top and bottom triple clamp pinch bolts.  Bounce it again to make sure it moves easily then snug the bolts for the fork brace and mudguard.  Bounce it again, then jack up the front wheel so it's off the ground,  can you grab the axle and twist it in the bottom of the forks.  If so, tighten the screw at the end of the axle and then snug up all the lower pinch bolts.  Bounce it again and if it's moving like it should go around and torque all the hardware starting at the top and working down.  Now you can bleed the brakes.

The front end is put back together and works like a dream, thank you Point-Seven-five.

Point-Seven-five wrote:You don't need a special brake bleeder.  A gallon jug, 2 feet of 1/4" clear vinyl hose from the hardware store and a 7/16 or 11mm wrench is all you need.  I like to use an old windshield washer fluid jug because I can stiick the hose down into the handle so it doesn't pop out.  Fill the jug about 1/3 with water so it sits solid on the floor and after you pull the cap off the bleeder, put the closed end of your wrench on the bleeder and put the end of the hose on it.  


I tried to bleed the brakes this morning using a method my brother suggested of holding the brake lever in and opening the bleed valve and letting the brake fluid jet out. I did this 3 times on each side and was feeling pretty confident that all of the air was out. I took the calipers off again, pushed the pistons out yet again and hoped that things would be back to normal. Unfortunately, no dice - same deal, 4-5 pulls of the brake lever and pads were touching disks again.

At this point, do I need to pull the calipers, pop the back off and remove the pads to get access to the pistons and see if there's a ton of build up on them? Is there something else to look for/try before I do that?

Cheers


__________________________________________________
1988 K75C
    

Point-Seven-five

Point-Seven-five
Life time member
Life time member
Is the brake fluid you are bleeding nice and clear?  Anything less than clear as water is old and needs to be flushed out.  

I guess you need to remove the calipers and get the pads out.  Are the pads worn level?  How much pad is left?  I use inexpensive Kevlar organic pads I get on eBay from a seller named D2Moto.  A full set, front and rear, goes for $18 with free shipping, so there is no reason to try to make my brake pads wear down to the backing before I replace them.  They are also very gentle on your rotors.  

With the pads out, clean around the pistons with CRC Brakleen using your wife's toothbrush(don't use yours because the stuff tastes terrible).  As long as the pistons aren't leaking I would avoid disassembling the calipers.  The reason is that many who have done so have had problems getting them back together.  After a good cleaning lubricate the piston where it rides in the caliper body with brake lubricant(not regular grease!).  Carefully work it into the gap.  Reinstall the pads being careful not to get any lube on the pad surface.  Put a light film of lube on the pins that hold the pads in place.

Put the calipers back on the bike and bleed them again(you can't do that too much).  Then go for a short ride.  After a 1/4 mile or so check the caliper to see if it's real hot meaning the pads are dragging.  If so let them cool and go another short distance and check them again.  As long as the pads aren't dragging and heating up the caliper and rotor so you can't touch them you are probably okay.  In normal use, the pads only back away from the rotor a couple thousandths of an inch, which is just enough to let the wheel turn freely.


__________________________________________________
Present:
1994 K75RT
1991 K100RS


Past:
1988 K100RS SE
1994 BMW K75S
1992 BMW K100RS
1982 Honda FT500
1979 Honda XR185
1977 Honda XL125
1974 Honda XL125
1972 OSSA Pioneer 250
1968 Kawasaki 175
    

nullfox

nullfox
active member
active member
So, some level of catastrophe it seems.

I pulled one of the calipers and removed the pads. I gave the brake lever a few pulls to get the piston to come out so I could see if there was any rust or build up that would be preventing them from moving smoothly. First thing I noticed (and maybe I'm wrong here) was that only 1 piston was moving. I was under the impression there are pistons on both sides that should move in and out?

Anyway, once I got at least one piston a out a bit, I tried moving the rubber dust cover on the piston back to see if there was garbage buildup - it looked pretty clean, however, I noticed that it started to leak brake fluid from around the piston, which it wasn't doing before. A few more pulls of the brake lever confirmed that it was absolutely dumping brake fluid out around the piston seals (I assume).

At this point, I'm thinking I'm going to get a couple caliper rebuild kits and possibly a MC rebuild kit and just rebuild all of the possible moving parts in one go now that all of the existing brake fluid is on the shop floor.

I'm not sure what if else, is another option at this point.


__________________________________________________
1988 K75C
    

Laitch

Laitch
Life time member
Life time member
nullfox wrote:. . . I'm thinking I'm going to get a couple caliper rebuild kits and possibly a MC rebuild kit and just rebuild all of the possible moving parts in one go now that all of the existing brake fluid is on the shop floor.

I'm not sure what if else, is another option at this point.
If the master cylinder is pumping fluid, leave it alone.


__________________________________________________
1995 K75 81,000 miles
Brake calipers, handlebars and fork rebound, oh my... Usa-lo10
    

Point-Seven-five

Point-Seven-five
Life time member
Life time member
Based on your experiences up to this point, I would very, very, very strongly suggest that you look for a couple used calipers on eBay.

https://www.ebay.com/sch/i.html?_from=R40&_trksid=p2505460.m570.l1313&_nkw=bmw+k75+front+calipers&_sacat=0


__________________________________________________
Present:
1994 K75RT
1991 K100RS


Past:
1988 K100RS SE
1994 BMW K75S
1992 BMW K100RS
1982 Honda FT500
1979 Honda XR185
1977 Honda XL125
1974 Honda XL125
1972 OSSA Pioneer 250
1968 Kawasaki 175
    

nullfox

nullfox
active member
active member
Laitch wrote:
nullfox wrote:. . . I'm thinking I'm going to get a couple caliper rebuild kits and possibly a MC rebuild kit and just rebuild all of the possible moving parts in one go now that all of the existing brake fluid is on the shop floor.

I'm not sure what if else, is another option at this point.
If the master cylinder is pumping fluid, leave it alone.

It seems like it is? At least when I opened the bleed valve and pulled the brake lever I was getting fluid coming out, though I don't know the best way to test for fluid returning. Is there an accepted way to test that the MC is working completely?


__________________________________________________
1988 K75C
    

Laitch

Laitch
Life time member
Life time member
nullfox wrote:Is there an accepted way to test that the MC is working completely?
There sure is. Restore the condition of your calipers, mount them into position, bleed your brakes then when that's done, squeeze and release the brake hand lever. If the brake pads grab the rotor then release it, you're golden.  cheers


__________________________________________________
1995 K75 81,000 miles
Brake calipers, handlebars and fork rebound, oh my... Usa-lo10
    

Point-Seven-five

Point-Seven-five
Life time member
Life time member
Once you've replaced your funky calipers and tested the M/C, it's pretty easy to open the orifice with a fine wire going in through the reservoir if you still have a problem.

Again, based on your past experiences with this bike, leave the M/C alone until you know for sure it has a problem. Let sleeping dogs lie.


__________________________________________________
Present:
1994 K75RT
1991 K100RS


Past:
1988 K100RS SE
1994 BMW K75S
1992 BMW K100RS
1982 Honda FT500
1979 Honda XR185
1977 Honda XL125
1974 Honda XL125
1972 OSSA Pioneer 250
1968 Kawasaki 175
    

nullfox

nullfox
active member
active member
New (used) pair of calipers ordered - once they arrived and I get them hooked up, I'll let y'all know how it goes.

Thanks all for the help.


__________________________________________________
1988 K75C
    

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