Thanks --

Question -- I believe that the water is going down the return to fast which is causing the vacuum. Wouldn't the vacuum be greater on the return line? I was thinking I needed to move it down on the supply line where their would be more pressure to keep it closed.

I was also considering putting a ball valve on the return line and restricting return flow to minimize vacuum -- will this work -- or is that a bad idea.

I'm just trying to figure this stuff out. (especially - since I'm paying someone to get up on the roof - I want to get it done as short as possible).

BTW -- tphaggerty -- thanks for all your experiences and recommendations.