What do you mean self priming cyclinder?
My cyclinder fills itself from a header tank. open vented
Do you think having the pump on the return is causing the cavitation?
Having just fitted my HW/CH system with new programmer and 3 port valve( honeywell V4073A fitted on the flow) all working fine, except I get bad cavitation noise inside the 3 port valve when it is in the AB mid position feeding both HW and CH. I have tried turning down the speed of the pump...