Baxi Bermuda BBU. 3 way valve. Feeding 9 rads in Bungalow. HWT in loft space, expansion tank in loft space. Replaced 5 year old Pump due to \\\"carbon growth/fur\\\" on blades (making clicking noise). Flushed system out. Removed each rad and pressure flushed out each one. Replaced rads and powered up system. Now system runs for 30 minutes then the pump is full of steam/hot air and circulation stops. During 30 minutes no rads heat up. When opening distant drain off tap, hot water rushes to drain off point. Close the drain off, then the pump runs for 30 mins after that it then needs to be bled. No apparent leaks visible all joints dry, no hissing or steam from BBU. Thoughts are a blockage but when you open bleed points the hot water runs well. When selecting hot water only....it does not pump sufficiently to lift boiler hot water HWT in loft, pump is rotating in right direction and installed correct way up. Any ideas? What is the best way to test if air is getting into system? Where is most likely point of problem?