Clutch Hydraulic line air ingress

Hi all, I need a bit of advice re clutch hydraulic line on my 2009 superb. I am getting gradual air ingress into the line overtime to the point I cannot put the car into any gear. Solution has been to bleed the clutch line and get the air out and there is no brake fluid loss as such I don’t think it is a leak issue + I am able to drive the car for few weeks between the bleeds.

I had the master cylinder and its seal replaced recently but this did not help. My money is now on the slave cylinder but given it is concentric one it is mounted inside the gear box as such it is a big ish job to do so wanted to see if someone else had similar issue before I go and spend anther 500 pounds or so unnecessarily. Thanks for any tips.