The forge doesn't have a diaphragm just a spring and an alluminium piston. as mentioned you can get different springs and shims. They used to list a full kit for about a tenner.
It doesn't explain why it does it with a standard valve though. Its as if you have too much vacum on the engine side when the throttle is closed compared to the levels on the intercooler side of the throttle.
Did u check all the intercooler pipes and connections and breathers.
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