Hydraulic clutch issue

[safe]

I have modified my pedalbox with a 993 master cylinder so I can operate the 964 slave cylinder on my g50 box.

However, I have problems with air... I can bleed the clutch so it works, but if I just let the car sit 2 weeks somehow air gets in the system and I can't engage any gears, this has happened 3 times now.

I don't see any fluid leaks, can air enter anyway, or can it be trapped air I haven't gotten out?