Can you retest water system with head gasket test chemical and see if it turns colour.

If you have definatley got all the air out when bled and it keeps getting more air in still sounds like compression getting in or exhaust gases.

Has it got a water cooled egr valve? have had these give similar symptoms to cracked heads /head gaskets ect.

Pressure testing of heads doesnt always show a fault, its nowhere near the same as running pressures.

just a few ideas .