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To hold, it has to have a check valve in it near the manifold. But it won't hold forever. The vacuum switch in the dash is leaky by design and so is the central axle disconnect by design if you...

If it has no leaks, then there is a leak in the lines somewhere

That is generally caused by a vacuum leak or more often a bad vacuum reservoir located behind the front bumper under the battery area. Low vacuum will cause the HVAC to default to defrost