Originally Posted By: George7941
...the slave cannot suck in air unless it is also leaking...
In my experience, with the high friction present in a concentric cylinder that is not universally correct and it's due to transient conditions that we don't normally consider.
After releasing the clutch pedal, due to crank end float the slave gets pushed back slightly more after a short time, returning fluid to the reservoir via the master cyl. On the next actuation, because the master cyl initially uses a small amount of stroke to close that clearance, on return the slave stops pushing back fluid after the pressure plate spring is fully relaxed since there is no return spring other than the pressure plate. A vacuum results before the master cyl is able to reach the point where it is open to the reservoir. If the slave seals are getting worn they will not be able to resist that vacuum and will pull air in, giving you a soft clutch.
The test for this is that the clutch system bleed holds with engine off, but fails when you start the engine and actuate the clutch a few times.
OP's issue sounds simply like there is still air stuck in the slave.