Hello there,
My 2004 stick(sick)-shift nissan altima has a clutch problem. Randomly - (especially when I use clutch frequently) the clutch sinks all the way down to the floor and I get only an inch of clutch. Once this happens, it will stay on for a while and it becomes extremely hard to shift gears and drive in the traffic. Giving the car a day of rest will fix the problem. No repair shop (including the dealer) could diagnose it.
Any ideas ?
The clutch master cylinder might be leaking internally once the engine heat gets it hot.
To determine if this is the problem, carry a bottle of water in the vehicle. The next time the clutch starts acting up, pull the vehicle over, open the hood and pour the bottle of water over the clutch master cylinder. If the clutch function returns for a while, replace the clutch master cylinder.
Tester
The only other thought I can come up with is that your fluid in the hydraulic system is contaminated and needs to be flushed and changed. It does attract water over time, and heated water turns to steam, which compresses easily and makes an hydraulic system not work. This is really thinking along the same lines as tester, but the overheated portion could easily be at the slave cylinder down by the clutch itself. It would not be unusual to need to replace the fluid (in fact, you are about 3 years overdue).
Thank you both for sharing your thoughts. I will try the “water treatment” next time. But honestly I don’t know where the clutch cylinders are located and how they look like 
Can you guys point me to a picture or provide a description?
I don’t think there is a leak. The cylinders were recently inspected and fluid was replaced. AAA -where I do the maintenance- said they don’t see any evidence of leakage.
Thanks
~b
Open the hood and on the firewall in front of the driver you’ll see the brake master cylinder and the reservior. Look on the left of the brake master cylinder and you’ll see another smaller cylinder with a smaller reservior. That’s the clutch master cylinder.
Tester