Hmmm try the G.A.S. procedure first
Let me guess you bought the Vaico valve from ECS and not the Genuine one correct? I would probably send it back and get my money back if its bad and buy the rebuild kit with new and improved flap, parts, and O-ring seal from
G.A.S. Home Page - I rebuilt my OE DISA valve and its been working great for years.
I would say put the rebuild kit on the Vaico valve if that is what you bought but I dont know the quality compared to OE - also if you go on G.A.S. website you can try another troubleshooting procedure - hold DISA flap closed and finger over vaccuum pot hole, then release flap - it should only flap back open like halfway