Re: GMC 4000 rear axle vacuum shift
While you're shifting, the clutch is pushed in so the engine should be at idle, the manifold vacuum should be around 15"Hg (with a healthy engine). This vacuum is applied to the control valve then passed on to the shift chamber (shift chambers should see the same as manifold vacuum). I see no regulator in the system. As long as the is no vacuum leak in the system it should work properly. Also make sure the air filter for the control valve is good. Any restriction (dirty filter or kink lines) would make the shift slower or not at all.
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