How can I fix this besides replacing the whole master cylinder.

 

Take it off of the bike, soak it in a cleaning solution, strip the old paint and lightly sand the body down being careful not to get any grit into the body, tape it off and repaint it. Or buy a new one.

 

check the cap to see if is flat ,get new gasket don't over tighten

 

This was unfortunately a common issue on many HD models over the last few years.

If you are under warranty or have an extended service plan, this would be a covered repair. I would be very careful trying to clean up and repaint. Any debris or solvents that get into the piston and bore will likely damage the piston seals and require rebuilding.

At that point, you are almost better off replacing as they have revised the master cylinder, cap and seal design a few times and the recent kits are much less likely to leak again. Expect this to reoccur within a couple years if you clean up and repaint.

 

Hello and welcome.
I see a picture but minimal information seems to be provided.
No year or model.
I will take a guess it is a clutch master cylinder.
In the past when this happened with a Harley motorcycle that was out of warranty the least expensive solution was a new gasket and paint.
Replacement of gasket, light sanding and then tape area and surrounding parts of bike.
Rust-oleum satin black spray was the closest in color.

 

Install a new chrome one.

 

Kuryakyn has a chrome cover that will hide all that uglyness and it just slips over your master cylinder. Good stuff

 

It's because you overtightened the screws and squeezed the gasket. There is no pressure or anything. The screws only need to be tightened a few in/lbs.

Jim