Every now and then the lever goes slack or doesn't return back to starting point when changing gears. Especially if I'm changing quickly. It's not consistent. The pressure comes back on its own again.
I've bled the system and it still happens sporadically.
I have aftermarket levers on since new (bike has 21000 Kms on now) and the pin is seated properly.

My assumption is that my master cylinder seals are on their way out. Is there a way to test to make sure?

I wonder IF you have a bit of air entering the line just enough...Is it doing this when the engine is 'cold'?Like within the first 10 miles or so?I had one that worked fine until the fluid heated up...I ended up replacing the whole unit.This is a tricky one for sure.Any seeping at the bleeders..or around any fittings?Fluid still good?You SURE all the air is out of that line...sometimes it catches down at the lower bleeder area in there...fine bubbles...which end up travelling upwards...straight to that master.

"The pressure comes back on its own again."...yeah,could be it hits that piston in there,can't compress,then in a bit,it travels up into the res.Just some thoughts.

* Last updated by: Grn14 on 12/18/2016 @ 9:31 AM *

As Grn mentioned, any leaks outside the master? Then no for this one visual test. The next is the lever pivot lubing. Might that hangup being dry maybe?

The next is the lever pivot lubing. Might that hangup being dry maybe?

I have noticed slightly lower pressure in the lever after not riding the bike a while. Never had it get worse after start up.

I have aftermarket levers on since new (bike has 21000 Kms on now) and the pin is seated properly.

It's a long shot but could the soft brass bushing the push pin seats in have worn deeper? I could see that adding a couple mm to the travel of the push pin. ...? The clutch gets pulled full throw all the time. The brake lever parts probably get a lot less wear.

I'd try OEM lever back on while you peek at that bushing. If you rule out problems with the lever and contaminated clutch fluid, the MC and slave are all you have left to worry about.