You should have had one in the original setup for the as part of the overboost protection. It should have 3 fittings for vacuum lines. When power is removed the internal switch opens and allows the air to pass through. Think of it like T intersection on the inside.

I have mine routed from the vacuum booster to the back of the IP with the overboost circuit in the middle. I left one of the fittings vented to atmosphere. When power is applied (via ignition), the valve closes to the brake booster line and the residual vacuum (between the valve and the IP) is vented to atmosphere, while the line to the booster is sealed and holds vacuum. When the power is removed (key off), the valve is closed to atmosphere and makes a clear way between the IP and the booster. I've been using the brake booster as the vacuum tanks since only a small amount is needed on shut down the engine.

Clear as mud? Hope that helps.

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'85 300D ~ 381k, HD Bilstien shocks, 27.50x8.50x14R General Grabber AT2 tires, 4 E-Code headlight upgrade with 90/130w bulbs, boost turned up, new timing chain, and injectors. SOLD

'85 CJ7 ~ OM617 swap, Tarus electric fan, T5 trans, Dana 300, 4.88 R&P, Mile Marker locking hubs, ALDA removed, AMC 20 rear disk brake conversion, Aussie locked with 33's and 5" Rough Country lift.