Here is something to think about.

I recently brought a 1990 GTR1000 with 19,000 miles but it had not been used for 6 years or so. After a new battery, full service, carb clean etc..etc it passed its MOT two weeks ago. 
Last week when riding it the clutch started slipping so I popped in my mates bike shop to order new plates. When my mate had a look he said there was no free play at the clutch lever. Hydraulic clutch levers don't have any adjustment so he investigated the problem. It appeared that the fluid was being pumped to the slave cylinder OK but was not returning to the master cylinder properly when the lever was released. This caused pressure to build up in the slave cylinder and as a result after it had been operated a few times the build up of pressure was causing the clutch to slip. 
They had never come across this problem before so the fluid was changed and the master cylinder stripped and cleaned. All parts appeared serviceable and all worked well when it was reassembled. 
I went to Wales camping on the bike the weekend and the clutch started slipping again after fifty or so miles, no free play at the lever again! I woke up in the morning to find free play and everything OK until the lever had been used a few times and the pressure built up again.
I hope I haven't done any damage to the clutch plates after doing a couple of hundred miles with it slipping over the weekend. I have brought a replacement master cylinder on eBay last night so will fit it when it arrives and see if it cures the problem. If not it's back to the drawing board. 

Any ideas?
Joe                                                                                                                                                                               

Check pipe work could have small blockage that gets though under pressure then slow release overnight                                                                                                                                                                              

Interesting problem. Has the bike got aftermarket levers fitted? sounds like the piston of the master cylinder could not be returning fully to its home position. So not allowing the pressure to reduce as normal.
cheers Alan
                                                                                                                                                                             

Hi Joe.   It sounds as though you may have a fault within the clutch unit itself, something sticking not allowing the clutch to fully engage, not pushing the slave cylinder piston fully back into the cylinder, not returning the fluid to the reservoir, holding the pressure in the system.   But an overnight period of no use allows the spring pressure in the clutch to engage again pushing the slave cylinder piston in and returning the fluid back to the pot ????????.    I hope that made sense to you, cos it made me think a bit   I've known this to happen with truck clutches before when the mechanical components start to seize.   Just an idea to chew over.   Hope you soon sort it out.   Regards Dave                                                                                                                                                                              

flexi hoses can collapse inside, fluid can pass under pressure but not return fully. also had a similar thing on brakes on a Cadillac, turned out to be a tiny hole in the master cylinder was blocked and not allowing fluid to return to the reservoir, next time it does it open the bleed screw and let the pressure out (be ready for the fluid to squirt out) it should get you going again. it happens because when the fluid gets hot it expands and if it cannot get back to the reservoir it will hold the clutch cylinder slightly open

Thanks guys
Some possible explanations here, I tried looking at it today.
I pumped the lever continually until I lost free play in the lever again then loosened banjo bolt at the master cylinder. The pressure was gone in an instant and free play returned. I hope this is telling me that the pipe is clear and the fault is the master cylinder itself. But still not sure?
Hopefully the replacement will arrive tomorrow so I can try that.

Joe                                                                                                                                                                              

yep, master cylinder not allowing fluid to return, in the master cylinder there should be a larger hole that allows fluid through from the res into the cylinder bore, there should also be a small hole that allows fluid to return, it could be bllocked or the piston in the cylinder is not returning fully. you can usually see the two holes when looking down into the top of the res, the small one will be nearer to the banjo end of the cylinder and should be easily cleared with a small drill bit or similar pushed through it.

hiya pojo another thing to have a look at is the fluid itself ....some places sell it labelled as "motorcycle clutch and brake fluid" with no dot number....i have an old zx10 b2 which uses the same system ( the gtr being the detuned version ) i use dot 3 in mine and have no problems ... you should also check pipe routing around the handle bars for tight corners and the original equipment is a steel alloy pipe that runs from behind the fairing around the side of the engine down to the slave cylinder to make sure it hasnt been flattened or kinked anywhere                                                                                                                                                                              

Fitter replacement clutch master cylinder today and it appears to have done the trick. I will see what happens over the next few days.                                                                                                                                                                             

Thats good hope it stays that way
                                                                                                                                                                             

Clutch working fine at the moment, keeping my fingers crossed!