Hello everyone, finally got my k100 up running after a year I purchased it, fuel pump was busted and then the ground wire was corroded so the bike wouldn't even crank. Took a while to diagnose and found out it was simply the ground wire.
Moving on to rebuilding the front master cylinder, it has been leaking for a while now, and I finally got time to fix it. Took everything apart and was wondering whether this is still salvageable.



The surface where the reservoir and the cylinder meet (where the oring on the reservoir sits) has a lot of build up, very hard to clean off, not even sure what it is. There's also build up in the cylinder, paper towel and brush has not worked. At this point should I try to clean it or should I just source something used? How should I be cleaning it if it's still can be saved?

Thanks in advance!

duck wrote:http://www.kbikeparts.com/classickbikes.com/ckb.tech/0.ckb.tech.files/master.leak/master.leak.htm

Thank you duck! I'll take a look

Be sure to get that area between reservoir and M/C properly clean and fit a new O ring. I see often the M/C repair kit does not include that single O ring and any leaks will remove paint from tank and fairings  ........ Ask me how I know

We have a local guy that fits stainless steel sleeves and makes a new piston. Last time I checked it was $200.00 Au once done the replacement kit is around $10.00 Au. He tests and Guarantees his work and does ship overseas.
Regards Martin.

Michael Sydney wrote:Be sure to get that area between reservoir and M/C properly clean and fit a new O ring. I see often the M/C repair kit does not include that single O ring and any leaks will remove paint from tank and fairings  ........ Ask me how I know

Good point.  Through experience I learned long ago that it's best to just leave the reservoir in place on top of the master cylinder if it's not leaking.  Once you disturb that O-ring it will usually start leaking until replaced.

Michael Sydney wrote:Be sure to get that area between reservoir and M/C properly clean and fit a new O ring. I see often the M/C repair kit does not include that single O ring and any leaks will remove paint from tank and fairings  ........ Ask me how I know

Yea, I should probably buy an entire unit as the aluminum part where the Oring sits in is pitted from oxidization as well

MartinW wrote:We have a local guy that fits stainless steel sleeves and makes a new piston. Last time I checked it was $200.00 Au once done the replacement kit is around $10.00 Au. He tests and Guarantees his work and does ship overseas.
Regards Martin.

It's a little hard sourcing a new master cylinder here in the US

Updates, bought an ultrasonic cleaner and damn it worked well, removed all the oxidization or whatever buildup that was,and exposed the pitted aluminum surface.




Put everything back and it started leaking right away, have a new piston but do you guys all think I should just source another master cylinder?

Go for it!
https://www.ebay.com/itm/Bmw-K75-K100-Front-Brake-Master-Cylinder/223757974909?hash=item341903bd7d:g:iXgAAOSw92Jd1GZH

brickrider2 wrote:Go for it!
https://www.ebay.com/itm/Bmw-K75-K100-Front-Brake-Master-Cylinder/223757974909?hash=item341903bd7d:g:iXgAAOSw92Jd1GZH

Thnx! saw this one too, by looking at the cylinder it seems like the paint is corroded, which probably means there was some leaking too?

Smearing silicone grease around the entry to the master cylinder helps reduce the chance of corrosion. Packing silicone grease under the rear master cylinder rubber boot reduces the chance of corrosion via water ingress, and helps lengthen the life of the boot.
Regards Martin.

The m/c offered on Ebay doesn't look too bad to me. However, buying on that site is always something of a roll of the dice.  But, the seller does claim it's in very good shape and his/her rating on EBay is 100% satisfaction. 
Ya pays yer money and takes yer chances.

The M/C listed on eBay is marked "ABS". I would assume this means  it is not compatible with the OP's 1985 bike?

MartinW wrote:Smearing silicone grease around the entry to the master cylinder helps reduce the chance of corrosion. Packing silicone grease under the rear master cylinder rubber boot reduces the chance of corrosion via water ingress, and helps lengthen the life of the boot.
Regards Martin.

Thanks for the advice!

brickrider2 wrote:The m/c offered on Ebay doesn't look too bad to me. However, buying on that site is always something of a roll of the dice.  But, the seller does claim it's in very good shape and his/her rating on EBay is 100% satisfaction. 
Ya pays yer money and takes yer chances.

it's kind of surprising to me that there are only two related sales on ebay