If you replaced the master one year ago, I would not mess with the master right away. I think that this is the most time consuming of all the problems that can occur with the clutch accuation system, so be glad that this one is probably not on the table now.
Jeff Aronson above has some useful input. I too, have went through the "clutch saga". My symptoms were exactly as you describe.
If I understand your description of the problem, I would recommend your next step is to: Get under the hood and look for hydraulic oil on or near the circuit. This will tell you if it is leaking. If you can't tell, clean everything really good with a product like "Brakleen" then operate the system vigorously for a while and check later. If there is oil, find the source and fix it; this will solve the bleeding off problem, If that doesn't fix it, go on to step 1) read the spec on the measurement from nut to nut and check it, and adjust it if needed. If that doesn't fix it, go to 2) re-bleed the circuit. If that doesn't fix it, go to 3) Change the flexi-hydraulic line. If that doesn't fix it, go to 4) Replace the slave cylinder. After 4, you will have replaced the whole circuit and after making certain that all the fittings are tight, this will have fixed your problem. I would be very surprised if your clutch is going bad, as the way you describe your problem, it sounds as if there is a variance in the symptoms from when the car is cold to when the car has been driven for a while (this is exactly what mine was doing). The only explanation that I could think of, is that the hydraulic fluid, when warmed, would decrease in viscosity and therefore decrease in it's ability to properly pressurize the circuit that accuates the clutch, thingy.
Nonetheless, if the symptoms vary, it would not be the clutch itself, as the symptoms would be consistent.
Hope this helps.
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