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[ribbert]

   Can somebody please explain to me how air in the clutch system can cause it to slip?
   Regards, Pete.

I can't.

[giantkiller]

I don't get that either. Unless like you said a plugged orifice.

[red]

Can somebody please explain to me how air in the clutch system can cause it to slip? The way I understand the clutch system, that would have the complete opposite effect. Air will not hold the pressure on the pressure plate, allowing slippage.
   Regards, Pete.

Pete,
Just guessing here, but I imagine that if there is air in the clutch hydraulics, and that air gets heated and expands, it could cause enough pressure to partially release the clutch.  Best bet: bleed the clutch system and see if things improve.  I saw a bleeder banjo bolt (~US$10~$15) for the top of the hydraulic lines.  That thing seems like it would be a great idea for the handlebar hydraulics, on both sides of the bike.  Locally, a good hydraulics hose shop may have them for you.

Red, I see where you are coming from, but if the air expands it should pressurise the system enough to expel the clutch fluid back though the port in the master cylinder. Maybe if the port was blocked it might have enough pressure to hold the pressure plate partially away from the clutch plates but this seems highly unlikely. (at least to me)
   Regards, Pete.

Pete,

Could well be, there.  This problem is so unlikely that I would not rule out any combination of causes.  I believe the simplest "fix" would be to bleed the clutch hydraulics, then check beyond that point only if needed.  I still recommend the banjo bleeder bolts, for any motorcycle with master cylinders on the handlebars.  I've had 'way too much fun trying to chase air bubbles DOWN vertical hoses, when air bubbles like to rise.