Like you, I'm suspicious that one of the new cylinders is leaking by. Check both for evidence of leakage, although it may be internal and not visible.

I don't think you have a problem with air getting back in, unless fluid is leaking out, which should be obvious by brake fluid somewhere on the outside of the circuit (master cylinder, line from master cylinder to slave cylinder, slave cylinder).

__________________
Respectfully,
/s/
M. Dillon
'87 124.193 (300TD) "White Whale", ~392k miles, 3.5l IP fitted
'95 124.131 (E300) "Sapphire", 380k miles
'73 Balboa 20 "Sanctification"
Charleston SC