sanivite 3 leak?

hi
I have a 4 year old sanivite 3 system pumping the kitchen waste water, sink, dishwasher and washing machine up to a sewerage pipe. The discharge elbow has blown off twice and others have fixed it. I am currently trying to obtain a watertight fit between the discharge elbow and the unit which fits inside it at the bottom which has the one way valve at it's top. There is a metal hose clip around the outside of the base of the elbow which should make the water seal. It leaks from this point when the pump activates, I just cannot get it to stop. Are you supposed to glue them in and then tighten up with the metal clip? I am using new parts and its no different to the old parts. Any help would be appreciated.