Just bought a K100rs (first bmw) which has been stood for ten years,rear brake cylinder had seized , but after a lot of wd 40 and fiddling seems to be ok.
But i am having trouble bleeding the brakes , i seem to have got rid of most of the air , but cant get much pressure , if riding at low speed in first i can lock the back wheel , but any real speed not much brake, am i better of replacing the cylinder as i have spent several hours bleeding!Is it possible to introduce air into the system while pumping the brake without any signs of a brake fluid leak,
Any advise thoughts much appreciated, thank you.

you may have hit the nail on the head snave ,,,,

it is quite common that the seals at the master cylinder piston may have become corrupted by pitting in the bore of the cylinder walls especially after sitting for so many years without excersize .

your probably aware that brake fluid loves to eat water vapours and then condense it ...result is pitting of the aluminium bore of the cylinder over the years .

dont know what to suggest at this point ....you could attempt to clean up the bore ....replace the piston rubbers ...etc ...have a look at motobins for kits of such ....there are other suppliers of those parts around too ...

important factors ....dont use the newer "dot" spec brake fluid ,,please try to get something that is compatible to dot 4 (i think dot 5.1 is the new equivalent ) the two fluids if mixed arent compatable and may affect all the rubbers in the system ....may take a few days .

be aware that every time you pump the master more than the usual movement action distance, you could be doing more dammage as the rubber crossing the pitted area could be wearing the piston seals .

a new master is often the only way to repair ...unless you strip and resleave the cylinder with new rubbers etc ....

obviously all the same applies to the slave (caliper) end

hopefully a good reblead of all the fluid in the system could help ?

just some suggestions

Thank you for the advise was not aware of newer dot spec fluid being a problem. Looks like i will have to spend the money and go for complete
cylinder rather than a kit. Cheers

You can use DOT 5.1 as a replacement for DOT 4, if the system been completely flushed/cleaned.
An advantage is that the DOT 5.1 isn`t hygroscopic.

But you CAN NOT USE DOT 5.0, which need gaskets with other material properties.

Inge K.