front disc brake issues

Sometimes the brake lever on my 74 seems to hit a mechanical stop as I pull it in. If I pull harder it passes by this spot with a noticeable clunk. It's very scary and I need to address it. The M/C is a sleeved 13mm bore that I bought from OB a few years ago. Can anyone suggest what parts I need to replace?

Thanks

It may be just the piston in the MC binding up, a slipping sleeve?
I've heard mixed reviews on the sleeved M/C's
What fluid are you using?

Do you keep the nub on the lever greased where it pushes on the piston? It must slide as it pushes... just a TINY amount of Moly grease makes a world of difference.

I just reread the post. You bought it a few years ago?
You might try flushing the MC down to the caliper out completely. Go thru a whole can of fluid if you have to and use DOT 5 as it won't mix with DOT 3 or 4