What does Valve, Assembly GM-2019488 do?

[pfarber]

Its the 'valve' that comes off the intake manifold for the hydrovac.

Is it a check valve that is open when the brakes are on, and closed when brakes are not applied? Vice versa?

Reason I ask is that the part on my truck had more of a restriction than a valve... a hole about 1/2 inch across... nothing spring loaded or that could possible shut off vacuum to the hydrovac.

[dr deuce]

Yes it is a check valve on the manifold.

Without it you would loose boost when you shut the engine off AND more importanatly it will let gasoline fumes pass to the hydrovac which can kill it.