Hi All
I hope someone can help with a very frustrating issue with the rear brake master cylinder.

Very Early 1983 BMWK100 13mm master cylinder

The rear brake caliper has had a service kit fitted and works fine, then I changed the brake hoses for stainless steel ones, again all working fine.

Then I noticed the rear master cylinder was leaking slightly, so I ordered a 13 mm refurb kit from Motobins UK  (Insane cost for what you get).

Fortunately I had another brake master cylinder to play with to see how it all went together, mmmm simple, but this cylinder was pitted so cant be used.

I dismantled the working cylinder, cleaned it up and painted it, then  carefully polished the inside to make sure it was not pitted, checked the new piston fit was good.

I have fitted the new seal kit to the 'working' master cylinder and fitted to the bike, but no matter how much I try I cannot bleed the system, there is no resistance to the pedal movement apart from the return spring.

I have even applied a vacuum to the bleed nipple on the caliper to see if I could purge the system that way.

I have disassembled and checked everything a couple of times and primed the whole system with fluid but still cannot get any brake pressure.

I am at a total loss, I rebuilt the front brakes no problem and done brakes on many other bikes but this has got me stumped...

HELP please

Any ideas?

Cheers
Mike

See here it's not worth putting a kit in  or honing.http://www.motobrick.com/index.php?topic=9364.msg128997;topicseen#msg128997
Regards Martin.

One of two things:

- you have the brake lever adjusted so that it covers the return hole in the master cylinder. Make it slack and see what happens
- polishing the inside of the master cylinder has taken it beyond sealing tolerance. It sounds daft but the wee things are quite finicky about things like that.

-Dai

That Magura master cylinder is a piece of sh!t that isn't worth screwing with.  Take Martin's advice and don't look back.  I've installed four of those ChiCom master cylinders successfully and they're all still working great.

The retrofit master cylinder has good pedal feel and enough stopping power to stop my fully loaded K75RT in city traffic without having to use the front brakes.  I couldn't do that with the crap OEM unit.

+1 on the Chinese m/c. I have one on my K100RT with heavy sidecar. About 6k milies in and its been great from day 1.

Point-Seven-five wrote:That Magura master cylinder is a piece of sh!t that isn't worth screwing with.  Take Martin's advice and don't look back.  I've installed four of those ChiCom master cylinders successfully and they're all still working great.

The retrofit master cylinder has good pedal feel and enough stopping power to stop my fully loaded K75RT in city traffic without having to use the front brakes.  I couldn't do that with the crap OEM unit.

Have you got any details of the after market master cylinder please, where to get it and whats it called please?

I go on eBay and search for "pit bike master cylinder" 

Here's one that looks like what I've been using:

https://www.ebay.com/itm/313595935068?epid=8049069129&hash=item4903c6395c:g:6CwAAOSwViZgmlyU

Spend some time reading the first couple pages of the Motobrick.com link Martin posted above to see how these master cylinders are adapted, and what kind of brake line fittings you might need.  The actual job to modify and install one just takes a couple hours.

First step is to buy the correct one. PS5’s link will take you to one with slotted mounting holes. This is what you need, not the version with the fixed circular holes because they wont match the foot rest plate holes.

Plain sailing after that (but make the push rod length right).

Here is one of several methods:

https://www.k100-forum.com/t14939-replacing-rear-master-cylinder-with-a-chinese-one#176877

Those master cylinders look like good solutions.

After spending ages trying to bleed the rear brake with the new piston kit from Motobins and totally failing, I nearly gave up.

Then out of desperation I compared the new piston to the old piston and the seals look very different, I very carefully cleaned the old piston and seals and noted they were not damaged just a build up of crud on then, re assembled the master cylinder with the original since new piston and seals, BINGO, 3 mins and I had good brake pressure, a couple more mins and the brake was fully bleed and working correctly.

So now really hacked off for paying out for the refurb kit.

As an aside note, I complained to Motobins about the kit they supplied and they agreed to send me a new kit or a refund. I chose a new kit.  So well Done Motobins UK for good customer service.

Word of WARNING the hose supplied with the Chinese master cylinder that I bought was not rated for brake fluid. According to my brake guy it was vacuum hose. I was doing a pre-ride check and I noticed moisture on the outside of the hose. As I gently wiped away the moisture the hose disintegrated dumping brake fluid over the garage floor.
Other than that I now have two spare ones in stock, as my previous ones ended up being borrowed. The replacement fitted in March 2017 is still going great.
Regards Martin.

Dai wrote:One of two things:

- you have the brake lever adjusted so that it covers the return hole in the master cylinder. Make it slack and see what happens
- polishing the inside of the master cylinder has taken it beyond sealing tolerance. It sounds daft but the wee things are quite finicky about things like that.

-Dai

I just rebuilt a front master cylinder off a BMW R1100RT.  Could not get the thing to prime or pump.  After much hair pulling and aggrevation, I discovered the return hold was completely blocked.  Ran a small drill thru and voila, primed and worked fine.  I've never had a rear one apart, so I don't know if you can remove the inlet hose piece to check the return hole, but maybe with the cylinder out, you could use a dental pick or similar to check if the return hole is blocked.