Looks like my K1100LT rear brake cylinder has seized and it doesn't look it's serviceable any more.

Anybody got a working spare for sale?

Or is there an affordable alternative part?

It's a 12mm Brembo part no 10 417 000.

Many thanks

As with any question that's fairly common, it's a good idea to use the forum's search engine when in need. I typed in 'rear master cylinder' and got this:

https://www.k100-forum.com/t12442-cheap-and-easy-master-cylinder-replacement

and these:

https://www.k100-forum.com/t9917-rear-master-cylinder-alternative
https://www.k100-forum.com/t10752-k100-rear-brake-master-cylinder

Try Motorworks and Motobins for more reasonably priced options other than the dealer, unless a fellow forum member has one in your neighbourhood.

It may not be dead.  If you have the screw on the brake lever adjusted too tightly and you apply the brakes fairly hard then the piston can get pushed in too far and get stuck on the set screw. Try loosening the small set screw on the side of the rear master cylinder to see if that frees up the piston.

And whatever you do don't ride a paralever K very far with a sticking rear brake.  It can heat the rotor up enough that it will ignite the rubber boot on the swing arm and then you'll have a nice little rubber fire to deal with.

@duck wrote:And whatever you do don't ride a paralever K very far with a sticking rear brake.  It can heat the rotor up enough that it will ignite the rubber boot on the swing arm and then you'll have a nice little rubber fire to deal with.

I know this to be true as I was on my newly-rebuilt R1100RS (essentially the same design Paralever swingarm/final drive as a K1100RS) and was out for the first ride, ripping along I-84 in Portland OR, where I lived at the time, when I noticed the bike gradually feeling sluggish. I pulled off the first available exit and had a quick squiz at the back end. The boot was smoldering, the disc as red as a Texas Republican on election day. With luck I was 100 yards away from a riding mate's house. I rolled into his driveway, banged on the front door and asked for a fire extinguisher. The flames were licking the swingarm and the boot had all but melted away. I gave it a quick burst of retardant. The rear disc was still glowing red. I turned the adjustment screw to free up the m/c and rode home to swap the boot for a new one, with my tail between my legs.

Yeah... I've had a rear OEM disc turn blue on LFB because I hadn't adjusted the m/c properly. There wasn't much left of the brake pads either. I was saved by a Triumph rider who just happened to have an 11mm spanner handy; I cracked open the bleed nipple to release the pressure and also rode home with tail between legs.

My brake cylinder has no screw. It's a 12mm Brembo cylinder... It's not moving at all...

Here's my flaming tale of woe:

I was selling a K100RS4V.  The prospective buyer came by for a test ride and, per my terms of test riding, he gave me the cash to hold while he test rode it.  I led the way on one of my other bikes for a while on the test ride since he wasn't familiar with the area so I led a course where he could get some turns in and a few freeway miles.

I let him the lead the way on the way back so he could set whatever pace he wanted.  After we got off of the freeway I noticed he was oddly speeding up and slowing down.  I wasn't sure what to make of it. After about a mile of funky riding like that he pulled off into a gravel parking lot.

It was then that I noticed smoke, then FIRE near the final drive of the K100RS4V. I was in a BIG hurry to put it out so I quickly threw my gloves off, grabbed a shop rag out of the tail of the bike I was riding and ran over to put it out.  Since the fire was on the left side of the final drive I approached the task from that side of the bike. 

Being left handed, that was the hand I used to try to put out the fire.  While my left hand and intense concentration were focused on putting the fire out I inadvertently grabbed the exhaust with the full palm of my right hand and burned the living crap out of my hand.  As one might imagine it was intensely PAINFUL.

Once I'd gone home to get my trailer and retrieve the bike with the burned up rear end, it took me several hours with my hand in a bucket of ice water and an entire fifth of Crown Royal until the pain was dulled enough for me to get some sleep that night.

@yamarider wrote:My brake cylinder has no screw. It's a 12mm Brembo cylinder... It's not moving at all...

Yes, it does have a set screw. If it didn't the piston would pop right out.

So, Duck, did the bike sell?!

@Two Wheels Better wrote:So, Duck, did the bike sell?!

Not to that guy obviously but when my hand healed up enough I put a new boot on it and the next guy to look at it snapped it right up.

Nope. No screw. It's off a 1994 K1100LT.

Holy cow.  I've seen oodles of K rear masters and I've never seen one without a set screw.  What the heck holds the piston in there?

Can you rotate the piston at all?   That will sometimes free them up.

What about the other side...?

@duck wrote:Holy cow.  I've seen oodles of K rear masters and I've never seen one without a set screw.  What the heck holds the piston in there?

Can you rotate the piston at all?   That will sometimes free them up.

Nope... completely seized... not moving a mm...

soaked it in WD40....

No idea what holds the piston in... maybe its a non-serviceable cylinder?

@Dai wrote:What about the other side...?

same same... no screw.

If you do end up replacing this with a new MC I would love to see the old one opened up just to see how it was made.  I, and others here, have had good performance from the very affordable non OEM master cylinders from AliExpress.

I had a look at the cheap alternatives on eBay and Ali, but they never provide details on piston size etc.

This one looks promising:

https://rover.ebay.com/rover/0/0/0?mpre=https%3A%2F%2Fwww.ebay.com.au%2Fulk%2Fitm%2F283279280766

It even has the outlet on the correct side... But no piston details...

Would it matter too much if piston is NOT 12mm?

@yamarider wrote:Nope. No screw. It's off a 1994 K1100LT.

It's quite puzzling that the 12mm rear master cylinder you've got is an oddball as the part number that's specified for that model (from 7/'93 on in the case of a 1994 K1100LT mit ABS) also fits K100 models, with and without ABS, going back to late '89 as well as various series K75 from about the same vintage, all of which have some form of adjustment to them. This part number continues through the end of K75 & K1100 production. Whether it comes from something else may be lost to history...

OEM part number 34 31 1 458 142 from online ETK

See here easy mod and cheap as chips.http://www.motobrick.com/index.php/topic,9364.msg90596.html#msg90596
Regards Martin.

See here easy mod and cheap as chips.http://www.motobrick.com/index.php/topic,9364.msg90596.html#msg90596
Regards Martin.

@yamarider wrote:Would it matter too much if piston is NOT 12mm?

They only ever used 12 & 13mm back there, ABS or non-ABS, so there might be a slightly more forceful pressure related to pedal application if it's a larger diameter and perhaps reverse if smaller. I wouldn't stray too far from the course.

@yamarider wrote:

@Dai wrote:What about the other side...?

same same... no screw.

Got to be held together with a circlip then.

Just to let you know - i chucked the old cylinder into the bin as it didnt seemed to be servicable and replaced it with a different used OEM cylinder. now we got rear brakes!

Was just checking and came across this thread

I just came across one of these brake cylinders as well, also has no screw. Was fitted on a 1994 k1100RS. They look to be not serviceable. I have replaced it with a spare 12mm one that can be repaired (with screw)