I have just drained down my central heating system and wanted to give it a flush before I change over to a new boiler. I wanted to put the cleaner in the expansion tank but it is not draining. I have managed to drain radiators OK. The expansion tank first feeds an AirJec II with vent back to exp tank. AirJec outlet goes to pump. system was working fine just before summer. Can these AirJecs get blocked?