On my 1985 K100RT, the front master cylinder began slowly dripping a little brake fluid on the right fairing pocket.   My rear master was leaking last year and I rebuilt that, so I guess I should of done the front at the same time.

Repair of Leaky Front Master Cylinder.  Remember to take pictures as you go.

First remove the bleed screws on the front calipers and pump the brake lever until all the fluid is pumped out of the master cylinder. 

Remove the brake hose from the master cylinder.

Remove the cover to the master cylinder and, using an absorbent paper rag, clean out any remaining brake fluid.

Remove the one Philips head screw holding the cover to the accelerator cable housing.

Remove the one Philips head screw holding on the right handle bar switch assembly and pull back the assembly just a little, letting it hang.

Remove the two cap screws holding the master cylinder to the handle bar and lift off the master cylinder.

Remove the plunger assembly by using a cir-clip plier to remove the  cir-cli,  then the plunger assembly can be pulled out. 

Clean all the gunk out of the master cylinder using brake fluid cleaner, cue tips, tooth pics and air gun so it is completely clean both where the plunger assembly goes into the housing and at the brake hose exit.

Clean all the gunk out of the area where the master cylinder assembly is inserted into the handle bar brake assembly.   I would not spray brake fluid in this area.  I used cue tips and tooth pics only.

Reassemble the piston into the housing liberally coating the assembly with brake fluid.  Reaassemble all the remaining parts in reverse order. 

Close the bleeder screws, add new Dot 3 fluid and bleed the brake lines until the brake lever has a firm pull.

 

Parts from Beemer Boneyard, BB 32722302356, 13mm rebuild piston assembly.  This includes the two new copper washers for reinstalling the brake line.

flyinglow wrote:add new Dot 3 fluid

NO. Use Dot 4 - it has a higher boiling point and a better hygroscopic point. Do not use Dot 5.0 - because it's actually an all-new silicon formulation and will bugger your brakes if mixed with 3 or 4. Crazily enough, you can use Dot 5.1.

[Edit] Updated because I made a stoopid error with the version numbers.

Be aware that the master cylinder repair kit probably doesn't include the O ring between reservoir and master cylinder. Worth getting these as a separate order.

Slow leaks from here are not good for paint on tank or fairing. Ask me how I know !   

NO. Use Dot 4 - it has a higher boiling point and a better hygroscopic point. Do not use Dot 4.5 - despite the .5, it's actually an all-new silicon formulation and will bugger your brakes if mixed with 3 or 4.

Hi Dai,

Is there a  4.5 ? As far as I am aware the silicon version is 5, with version 4 or 5.1 being polyglycol-ether based .

Mike

I sit corrected!! Yes, I meant 5.0 and that stupid confusion with 5.1. Someone should have been made to drink a pint of the stuff for that.