When I turn my motor off, I hear a slow but quite prominent 'psssshhhhhhhhh' sound, like something had strong vacuum on it, then when the motor was turned off, you hear the vacuum leak down again.

I believe it is the weird multi-orb vacuum reservoir thing in the centre console? That looks like a few light bulb sized globes connected together?

I'm not sure that's where it's coming from, but from that general area.

The hose that supplies the vacuum for the controls has a one way valve at the manifold.

The sound you describe occurs when that one way valve is defective. If you pull it off and you can blow air through it from either end you have confirmed that it is defective.

At the manifold? In the engine compartment? But you can hear it strongly in the car...I guess you just hear air escaping. Could you kinda describe where on the manifold it is or what it looks like?

Yup, this is a common failure item on old 240's! Sometimes they can be cleaned and restored to functioning in case you can't find a good used or new one....

jeeze, I went to my local, large automotive parts store, they can't even order one.

Google "volvo one way valve". Pretty common item.
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Post Back. That's whats makes this forum work.

Hold it under running water for a few seconds, shake it out, retest it. Saves a trip to the boneyard. Even saves a trip to the store. And saves money.

If that doesn't do the trick, pop it open and find out why. Then post back why you couldn't fix it.
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Art Benstein near Baltimore

Generally speaking, you aren't learning much when your lips are moving.

Try a boneyard first. If you have to go new, then try FCPeuro.com or Tascaonlineparts.com