Topic: Leaking MC

[derelicte]

I recently rebuilt the master cylinder on my cb750 and before installation back on the handlebars, realized I needed it hooked up to push out the caliper piston.  New kit was installed correctly with seals in the right direction and everything looked fully seated. 

I hooked up a piece of my old brake line directly to the caliper, added fluid again and bled it enough to pop the old piston out, and my newly rebuilt MC looked to leaking pretty badly.  This was done not hooked up to the handlebars (see below pic), so I tried to make sure I didn't pull the brake handle past its normal length of travel, but could that have been the reason for the leak?  If so did it mess something up?  Help appreciated..  Everything is still off of the bike and I'd hate to have to repaint the triple tree again do to another damn leak.

[derelicte]

[aperry]

Where is the fluid leaking from exactly?  If it's in the area where the lever meets the MC, then it could be that you've pulled the lever too far during the bleeding process.  There's a known issue during bleeding where if you pull the lever all the way back to the handlebar, then one of the reservoir holes can leak behind fluid the seal and then it'll drip out near the lever.  Yours wasn't mounted to the handlebar, but if you pulled the lever back more than an inch or two from it's fully extended position, then it may leak.  It should go away when your brake is fully charged up. 

Aaron

[CoachDoc]

I have the same MC on my 550, and the same problem, although the leak is minor. I control it by blotting the fluid that has oozed out after I ride with a paper towel. I assume there may be some minor scoring on the piston or cylinder wall that causes this. I view my choices as either live with it, repeat the rebuild on a used MC with  probably the same results, or go aftermarket which I don't want to do.

[derelicte]

Great to hear, hoping it was just from pulling in too far.  I did leave a superficial scratch on the very end of the bore (on the "button" side where the brake handle presses in) but I was pretty sure both seals are further in.  Thanks for the help, will post back when done.