Rather than rebuilding my rear master cylinder on my 87 K100LT, I figure a newer replacement might be a better bet. When scanning ebay it appears many units from newer BMW's appear nearly identical. The R1150RT in particular. The pricing is much more reasonable. Has anyone swapped one of these or similar units? As it is a rear brake, I suppose optimum braking isn't crucial, after all I have been living with a wimpy K100 version for some time now.

The manner in which the piston is pulled and pressed by the brake pedal is different from other Beemers, even some other model Ks. I think you'll find that if you want original equipment operation you'll need an OEM-type master cylinder.

TWB - I think BrianPen is right. The R1150RT rear master cylinder does appear to be the same one as fitted to the K100; my caveat is that the hole spacings may be different. This listing distinctly shows that it's a push M/C in similar fashion to the K100:

http://www.ebay.co.uk/itm/04-2004-BMW-R1150-R-1150-RT-ABS-R1150RT-REAR-MASTER-CYLINDER-BRAKE-8585-/351295869872?hash=item51cadd93b0

Interestingly, there's no rubber buffer between the actuating rod and the M/C piston.

This one shows the same bellows fitting as we have:

http://www.ebay.co.uk/itm/BMW-R1150RT-2004-Twin-spark-51-547-miles-rear-brake-master-cylinder-/161682225730?pt=LH_DefaultDomain_3&hash=item25a502fa42


BrianPen: like you, I hated the feel of the rear master cylinder. I threw away that stoopid rubber buffer and replaced it with a home-made solid stainless steel one. IMHO instant improvement in feedback and braking.

I could be missing something here, but aren't the mounting bosses on the R1150 M/C on the bottom of the unit whereas on the K bike master cylinders they are on the top.  How can they be interchangeable?

I have an R1150RT in my garage too. The rear MC from that wouldn't work on a K100/K1100 unless you replaced the footpeg mounts and brake pedal with something custom.

Ah - that bit of information isn't exactly visible in pictures!

A coupla months ago I looked at as many options I could for my project bike. In the end I ordered a new 13mm rear M/C. There wasn't gunna be an easy answer unless I went the (wicked kewl) BSK Speedworks rearset way, or other. I opted for standard peg plates and stock position....for now.

I've been running a Chinese rear master cylinder purchased of Ali express for $13.33 AU. Easy conversion some models may require a new hose.http://www.motobrick.com/index.php/topic,9364.msg90596.html#msg90596
Regards Martin.

I, too, am running those Chinese master cylinders on two of my bikes and have a combined 20,000+ trouble-free miles on them.  Probably the best $12 I have ever spent on my bikes. 

IMNSHO, when it comes to rear master cylinders, f*ck BMW.

I am having trouble finding the Chinese part, can you help me please?

This is the one I've got it actually feels better than the OEM. my Local brake shop charges around $50.00 Au to make up the hose if needed.https://www.aliexpress.com/item/Motorcycle-Rear-Foot-Hydraulic-Brake-Pump-For-Suzuki-Kawasaki-Honda-Yamaha-Refit-Rear-Brake-Master-Cylinder/32789801624.html?spm=2114.search0204.3.40.41e44493IWvqhI&ws_ab_test=searchweb0_0,searchweb201602_3_10065_10068_10130_10890_10547_319_10546_10548_317_10696_453_10084_454_10083_433_10618_431_10307_537_536_10131_10902_10132_10133_10059_10884_10887_321_322_10103,searchweb201603_51,ppcSwitch_0&algo_expid=5d6f8745-078c-40ae-821a-a72e6c30bdab-5&algo_pvid=5d6f8745-078c-40ae-821a-a72e6c30bdab&transAbTest=ae803_4
Regards Martin.

Ordered 12 Jan 20 expecting a "long" wait.
Delivered 24 Jan 20
Gotta be happy with that!!!

Just a warning for those contemplating fitting a Chinese master cylinder. DO NOT use the supplied hose between the reservoir and the master cylinder. It is not rated for brake fluid and it will fail eventually possibly dumping brake fluid over the rear tyre. The cylinder itself is fine and I believe it has better feel than the OEM.
Regards Martin.

Thanks a lot for the hot tip Martin.  Chinese special M/C on the way!

from s1000rr fits perfect you can buy cheap