forgive me if I am asking questions that have already been extensively covered in other topics but I have not been able to find anyone having the same problems as I have yet. My problem is this;

The rear brake pedal moves only a little but is really solid once it gets to its final position. With the bike on the centre stand you can very easily still turn the rear wheel and there is plenty of fluid in the reservoir.

I have only had the bike for a couple of weeks and have not actually had much of a look at this problem yet but will hopefully be doing so today. My plans are as follows - I have already opened the bleed nipple to see if there is any fluid coming out or even a change to the firm feel of the pedal and nothing. I am therefore toying with the idea of removing the hose from the calliper to see if any fluid is making it that far and then, if it isn't, taking it off from the master cylinder itself. While I cannot actually get any fluid through it seems pointless trying to refresh the fluid as it will not get all the old stuff out.

Any ideas or suggestions would be great.

If there is nothing coming out of your caliper/slave bleed nipple then it has to be your master cylinder and it sounds like its seized.
It shouldn't be rock hard but firm and a tiny, tiny bit squishy.
I'd be puting a new kit in both master and slave.
Cheers

+1........a easier solution would be to buy a new master, but  

Often when the piston is seized solid, the bore is corroded beyond repair.

Captain Holister wrote:If there is nothing coming out of your caliper/slave bleed nipple then it has to be your master cylinder and it sounds like its seized.
It shouldn't be rock hard but firm and a tiny, tiny bit squishy.
I'd be puting a new kit in both master and slave.
Cheers

This it the most likely. Since you will no doubt be doing a complete brake fluid change I would also open up the bleed nipple in the rear caliper just in case there is a problem there. If this bleeds fluid then a seized up caliper. But my money too is on the rear master cylinder and a new kit. I had the same thing and its a false economy only replacing part of if it has been seized for a while.

If you over-adjust/tighten the rear brake then hard application of it will cause the piston to freeze on the set screw.

I have already tried with the bleed nipple open and there was no difference (as I said in the original post) and am suspecting that the master is seized. Can't afford to buy a new one but have tracked down a repair kit already. Not ordered it though until I have chance to take it apart and see if it can actually be done or if it needs replacing.

 

duck wrote:If you over-adjust/tighten the rear brake then hard application of it will cause the piston to freeze on the set screw.

Suspecting this is not the case as there is a gap between the adjuster and the pushrod when the pedal is fully up. Stupid question though, is there actually meant to be a gap there when the pedal is up?

Grizzly wrote: is there actually meant to be a gap there when the pedal is up?

Yes, a small opening......so you can be shure that the piston returns fully.

I set my pedal so that there is about 2mm of free travel.
How far does the pedal go down before it comes to a stop, They don't have much more than 30mm travel at the best of times.

Well yesterday, after all the running around I had to do, I separated the union from the calliper and pressed the pedal - nothing! I then worked further along the line towards the master cylinder and took the flexi-hose off and again pressed the pedal - nothing!

  

I then fully removed the rear master cylinder from the bike and took a look at it. Well firstly the rubber around the pushrod has split and it looks like this has allowed water/dirt to get in and seize it. But there is also slight damage to the pushrod tube itself where it looks like someone in the past has been messing around with it after probably having similar issues so I have come to the conclusion that I will need to be sourcing a replacement master cylinder.

  

The question I now have though is this. Has anyone fitted a master cylinder from a different motorbike to theirs and had it working? I am sure I can more easily (and cheaply) find a good rear master cylinder and then fabricate a bracket to fit it to the bike and make it work. After all, I rebuilt a 1969 One-Ton Land Rover replica once in my garden so making a small adapter bracket should be a doddle!

Your thoughts.

If its not refurbishable, there's gotta be a second hand one around that would be a better option.
There are a few for a K1200 and R series for sale on ebay UK. They look like they might be similar. Someone here would be able to advise.
http://www.ebay.co.uk/sch/i.html?_odkw=&_osacat=177921&_trksid=p2045573.m570.l1313.TR0.TRC0.H0.Xbmw+k100&_nkw=bmw+k100&_sacat=177921&_from=R40

There's one for a K100 for sale in the US but freight might be a little pricey.
http://www.ebay.com/itm/1989-BMW-K100-RS-rear-master-cylinder-/171340024269?pt=Motorcycles_Parts_Accessories&hash=item27e4a949cd&vxp=mtr

Also you could have a look thru the Buy & Sell forum

Used - Brake master cylinder rear 13mm up to 10/88 | BRK54351 Fits K75S, K100
Larger Image	Magura master cylinder. The size is indicated on the body. See [url=http://www.motorworks.co.uk/vlive/Shop/Parts.php?T=6&Q= BRA54351] BRA54351 [/url] for new option	£60.00 No VAT payable Stock of this item varies daily. Please call to check availability and condition

This one is in Motorworks. You might need to check how much comes with it, as in does it include the piston and seals or does it need the rebuild kit as well. I just did my front brake and only needed a rebuild kit which I got from local BMW outlet.

Heaps of them here
http://www.ebay.co.uk/sch/i.html?_trksid=p3984.m570.l1313.TR0.TRC0.H0.XMotorcycle+rear+master+cylinder&_nkw=Motorcycle+rear+master+cylinder&_sacat=0&_from=R40

Just a quick update on this for people. I ended up buying and fitting an after-market brake master cylinder and when I took the bike for MoT it passed with no advisories. The rear brake works a treat and the tester said he was very impressed with how effective it was and such a neat job of the fabrication.

The master cylinder bolts in place by sandwiching an adaptor plate I made in place with the front one of the two bolts now being situated where the rear bolt would normally go. I made an adaptor which screws into the brake pedal so that the shaft from the new master cylinder pivots and the bore and stroke of the new item are the same as the original BMW part so braking force should remain the same. I have only had chance to try it a little as later on today I am off to take my module two of my bike test and will hopefully be passing and then getting the insurance sorted this afternoon. Following that, I will be going out for my first (legal) ride of my new bike.

Incidentally, I also ended up fitting a braided brake hose and using a couple of adapter unions and making a new brake pipe for the connection to the calliper itself.

Kaptain Holister wrote:If its not refurbishable, there's gotta be a second hand one around that would be a better option.
There are a few for a K1200 and R series for sale on ebay UK. They look like they might be similar. Someone here would be able to advise.

I have the same question about the K1200 rear MC but I see there were no answers. So, can the K1200 rear MC replace a leaking K75 rear MC (in a non-ABS rear brake setup) ?

Here is a guy who re-sleeves them in your area http://www.johnstuartpowerbrake.com/Master-Cylinders-Wheel-Cylinders.page
I have had about 5 M/C done over the years and all have been good and a stainless sleeve will outlast the bike.