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Can't find any info on holding vacuum but they should redirect air when vacuum is applied.
Check the out put of the pump those pumps would fail if they sat 2 weeks in the rain without running.
Pull off the vacuum line to the top of the valve with the engine running. There should be vacuum in the line. Install the vacuum line and inspect. No air should be escaping with the engine running at a steady idle.
Open and quickly close the throttle. A blast of air should come out of the valve muffler or air cleaner for at least one second.
If the truck is equipped with a electric diverter valve go to the next Step. If not equipped with the electric type go to Step 6.
Disconnect the harness connector from the solenoid and then turn the ignition on. Connect a voltmeter or test light between terminal A on the harness connector and ground.
If 10 volts or greater is measured or the test light lights, the solenoid is getting power.
If less than 10 volts is measured or the test light does not light, the solenoid is not getting power due to an open or short to ground in the wiring.
Measure the resistance across the solenoid by connecting an ohmmeter between the two terminals on the solenoid. Make sure that the solenoid is disconnected or a false reading may occur. The resistance of the solenoid should equal about 20 ohms.
If the resistance is less than 20 ohms, the solenoid is shorted. Replace the solenoid and the Engine Control Module (ECM), as required.
If the resistance is more than 20 ohms, the solenoid is open. Replace the solenoid.
If the valve must be replaced, use a new gasket at the valve mounting on the pump and torque the bolts to 85 inch lbs. (9.5 Nm).
I'll post back a link to check any other questions you might have