A likely culprit with intermittent vacuum system "failures" is the check valve that is inside the nipple where the metal line attaches to the vacuum pump. The retainer has a habit of breaking after 25 years or so. This allows the check valve components to rattle around and intermittently block the air flow into the vacuum pump. The nipple can be easily removed and inspected with the pump still on the engine. If it is broken, it will be necessary to fish the components out of the pump inlet.