Re-occurring air problem

About 2 yrs ago I was involved with a heating renovation in a home on Long Island. The home was heated using steam and there was a brand new Burnham gas steam boiler in use. My contract included converting the steam boiler to hot water, providing 2 new zones and running out a new feed and return to the 2 new fan coils. The fan coils, the blower units and all duct work was supplied and installed by the HVAC contractor.All that I was hired to do was the boiler conversion and the loop work. The boiler was converted using the Burnham factory recommended steam to hot water conversion kit. The two zones are set up using a 2tee manifold with a 007 IFC circulator on the supply side of each. The loop work is done in Wirsbo/Upunor pex tubing. The larger fan coil located in the attic is piped in 1" pex. The smaller unit is in the basement and piped using 3/4" pex. Originally each fan coil was supplied with taco 1/8" auto vents. They have since been removed and replaced with with manually operated coin vents. I have been back to this job at least 4 times in 2 years because the 2nd floor zone has become air bound. Each time I have tried to bleed the fan coil from its bleeder valve first and there seems to be no air in the coil itself. I have had to go down to the boiler and purge the loop from the purge valve on the return side. And each time I have removed a significant amount of air.The system has no visible leaks and I have even left the feeder off for two weeks with no pressure loss. Could there be a problem associated with either the IFC circulators or possibly the pex that is causing this re- occurring problem. On my last visit(this December) we installed a Taco "vari vent" style purge on the supply header. I am still not confidant that the problem is solved. Any thoughts ?