Hello, my name is Mike and I can help with this.
I think it is possible that your problems are with the slave cylinder now, or possibly even the clutch pressure plate (less likely).
A clutch slave cylinder can develop a leak that will allow air to enter the system. Bleeding the system or perhaps replacing the clutch master cylinder would remove that air from the system and make it work better for a time. But gradually, air would seep back into the system and slowly you would feel this problem re-develop.
I hope this is the type of answer you are looking for.
Thanks,
Mike
Glad to help. A simple test for them should be to bleed the system. If there was air in the system, and the pedal gets better...then there is a leak.
ASE Certified Technician
20 years experience, all makes and models.