I'm lookingf for a rear master cylinder for mt K100RS 1988 model.

Does any know if a master cylinder from an 85 model will fit?

I've been running a $15.00 Au Chicom master cylinder since early 2017 with good results. Depending on the set up on your Brick you may have to get a hose made. They now come in black and I bought a second one as a spare.
See http://www.motobrick.com/index.php/topic,9364.msg116179.html#msg116179
Regards Martin.

The bore size changed a few years into production. I don't think anything else did and the change was just to improve the feel of the brake.

I find that the Motorworks website is (unintentionally!) useful for checking compatibility over different models and years.

Thanks for the info

I have been running a Chinese one too for about 3000 miles very pleased with performance and price. Doesn’t take much to retrofit.

I've installed three, or maybe it's four, of the ChiCom master cylinders on my bikes over the past 3 years and put a total of well over 20,000 absolutely trouble free miles. 

Putting one on my K75RT actually made it possible for the rear brake to actually slow down the bike!!!

I've just installed this Chinese rear cylinder. 

It looks like you have used the reservoir supply hose that came with the master cylinder. Mine failed dramatically. I went to my local brake guy to get another piece of brake fluid rated hose. I showed him the failed hose and he said it looks like vacuum hose.
Regards Martin.

Yes it is the hose supplied. Thanks for the tip Martin, I'll replace it. I haven't had a chance to test it yet, I'm waiting on a banjo hose end adapter to complete the install. Cheers Liam.

I came very close to a disaster, I was about to go on a long trip and was doing a final check. I noticed a wet look to the reservoir hose so I decided to give it a wipe with a white rag. I just touched it very gently and the hose disintegrated and dumped brake fluid over the garage floor. If I had gone on the ride I possibly wouldn't have finished it as fluid would have ended up on my rear tyre. It pays to check and maintain your Brick.
Regards Martin.