That filter may actually be a filter for keeping oil vapors out of the brake booster, or a one-way valve depending on what you've got. Some vans even have both.
Since the brake booster is full of vacuum, once the engine shuts off, atmospheric pressure pushes any fumes in the manifold into the booster, where the rubber parts can be attacked over time.
You could attach a hand operated vacuum pump to the booster to see if it holds vacuum.
As for substantial vacuum leaks in out of the obvious places, try the charcoal evaporative canisters and their hoses. Those hoses frequently turn mushy and split wide open. The master canister (with the vacuum actuator on top) has a rubber diaphragm inside which can rupture, causing a very substantial vacuum leak. When you attach a hand vacuum pump to that actuator, it should hold vacuum.
Among my favorite stealth failures on vintage vans is the fuel tank cap. It has a breather valve built in. Driving at highway speeds, you develop maximum vacuum from the fuel pump. If the breather valve in the cap fails, you will starve the carb for fuel, and can literally collapse a tank. All you need to do to eliminate that as a culprit is to try opening the cap when you've got a stall and listen for a "whoosh" of incoming air to replace the built up vacuum. It's possible to drive with the cap partially cracked open too, though that does have some potential safety issues.
-It's been such a LONG TIME... BlueShift>>
1981 Dodge Ram B250 Custom Sportsman Maxi Van
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