Hi alan Now that you have removed the checkvalve, its very easy to see if its functioning well.
I think it has a little arrow or something on it indicating the flow direction. Check with a vacuum tester. The check valve should allow flow in only one direction. Its quite possible that it was blocked or malfunctioned so that it will now allow the vacuum to travel thru.

I have never had a failure so far so I have not done the troubleshooting first hand.
Seek the opinion of other members on this issue.

MVK

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1985 300D Turbo 179,000miles