I suggest fully-changing the brake fluid first, do a complete flush. You need the reservoir to have clean fluid in it and throughout the system. If your fluid is opaque, you are
way-overdue for a fluid replacement! Brake fluid is hygroscopic, it absorbs water. That lowers the boiling point of the brake fluid, which can in extreme instances cause you to lose hydraulic function. It also causes corrosion of the system, as ferrous-metal parts will corrode from the water content.
I recommend the 'reverse-flush' method to bleed the system, you use a syringe to push fresh DOT 3, DOT 4, or DOT 5.1 (I use DOT 4) from the clutch bleeder by the left footpeg up to the empty master cylinder. Directions below show how to build an inexpensive syringe for this purpose, and how to use it. I used to use a MityVac, but it sits lonely in my toolbox, used but rarely.
https://www.vmaxforum.net/threads/clutch-slave-cylinder-replacement.45011/
Measure the friction disc thickness for wear (2.8mm or 0.11"), check the 'steelies' for warpage (a piece of glass is useful) and 'bluing,' from overheating, if you have it, the color is obvious, and they need replacement; and scuff the 'steelies' up with sandpaper. That removes any glaze. If the friction discs are at the wear measurement or beyond, then new ones should help, yes. I'd consider replacement of the diaphragm spring if upon reassembly it still doesn't grab properly. I see no need for an aftermarket multiple spring pressure plate given your style of riding, nor a double-disc spring mod (adding two diaphragm springs together). I also recommend the OEM friction discs, every Barnett set I had, the friction discs would stick-together overnight, and I'd have to break them loose with the first clutch actuation in the a.m. (hold the front brake full-on, and release the clutch, stalling the engine, to free-up the plates).
PCW offers a heavy-duty diaphragm spring
[email protected]
You can also replace the innermost half-width friction disc with a full-width friction disc, and remove the wire (#9, below) which rides inside of the half-width friction disc. That assembly is Yamaha's version of reducing clutch reaction from poorly-matched engine speed and downshifting.
http://vmoa.net/VMX12-Service-Manual01.pdf page 191 of 405pp. of the .pdf; chapter 3, p.67 or 3-67 from the table of contents
