Clutch pedal pressure changing

Maxima Gurus, Please see the detailed problem listed below. The clutch pedal pressure varies during the day.
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97 Nissan Maxima SE, 5 speed, 184416 miles presently (no mods done on the car, one owner, bought new at Buckeye Nissan June 1997)
05/24/2008, 175 K miles     Symptom:    Could not shift the gears     Solution:    Mechanic replaced the Clutch slave cylinder     Cost:        $220 (parts and labor)
06/03/2008, 176 K miles     Symptom:    RPMs would increase but car would not go forward     Solution:    Mechanic replaced the clutch, pressure plate and throwout bearing     Cost:        $1005 (parts and labor)
11/17/2008, 184332 miles     Symptom:    Clutch pedal falls to the floor,     Solution:    Mechanic replaced Clutch Master Cylinder, bled the system     Cost:        $230 (parts and labor)
After all these repairs, the clutch pedal pressure is not constant. In the morning, when I get ready to go to work, the pedal offers very little resistance and as I drive around, the pedal pressure increases and becomes stiff. My work place is 15 miles South of my home. When I start the car back at 5 pm, the pedal is soft and it becomes stiff as I drive back home.
I have asked the Mechanic to check for leaks since this is indicative of a hydraulic leak somewhere in the line. I took the car back to him on Dec 1st and he thought the lines between Master/Slave had moisture in them.
12/01/2008, 184400 miles     Symptom:    Unstable clutch pedal pressure (varies) Solution:    Mechanic replaced the lines between Master and Slave cylinder and bled the system     Cost:        Has not charged me yet
The clutch pedal pressure still varies as I drive in spite of all these repairs. He insists there are no leaks anywhere. The clutch hydraulic system has been bled twice.
All repairs above were done by the same mechanic. He says he cannot pinpoint the problem. Is this a hydraulic problem or a clutch problem?
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wrote:

Not a Maxima guru but I don't think that matters as all clutch systems are basicaly the same.
Your problem as I see it now: First thing in the morning (when engine is cold) - clutch functions correctly.
As you drive and the engine heats up, the clutch becomes stiffer.
After work, when engine is cold the clutch is back to normal.
Appears to be temperature related.
Can't think how it would be a hydraulic problem, that system is nothing more that two hydraulic cylinders connected by a pipe.
If the hydraulic system was leaking, air would unter the system and the clutch pedal would be spongy (air in the system is compressing) and the spongyness would improve when you pump the pedal.
Must be a clutch pressure plate, disc, or throwout bearing problem which changes with temperature.
Clutch throwout bearing is binding on the gearbox shaft, or there is a problem with the lever which acts on the throwout bearing.
Al Moodie.
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r

forward

te and throwout

bled the system

en

and Slave

I had a similar sounding intermitant problem with an RX-7 and it was the clutch master. Maybe you got a bad clutch master. I hate these intermitant problems. Good luck man.
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