16mm master cylinder with a mirror mount?

I don't like the spongy feel of my 14mm master cylinder and dual discs on my CB550. (Yes...I am POSITIVE there's no air in the lines).

Can anyone tell me what bike might have a 16mm master cylinder with a mirror mount? I believe the Honda mirrors use 10mm thread.

Your brake lines may be expanding. My bikes brakes allowed me to pull the lever all the way to the bars, and still not have a full application. Bled well.

I changed to Stainless Steel lines, same master cylinder, same caliper, and now the brakes have a full application with the lever about halfway to the handlebars. The feel is better, control is better. I did both my 91 NH750, and the 76 CB360. Both have greatly improved brakes. Work it, in my book.

A large Master Cylinder diameter will actually decrease the hydraulic force you can apply to the brakes. They will feel firmer, but will have less stopping power.

Watch your rubber lines carefully when you squeeze the lever hard. I bet you can see them expand. If they are the original lines, they need replacement anyway. The rubber gets weak with age.

You looking for the master with the built in res or remote res?

+1 on the Stainless lines. They will make a huge difference.

Use a Suzuki rear clamp, GS 500 has mirror mount on the back so it will fit almost anything
There are probably others with same set up, I just don't know what.