Checking pressure retention valve

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Fig 1: Checking Pressure Retention Valve

Courtesy of VOLKSWAGEN UNITED STATES, INC.

Check direction of valve flow:

A) Union 1 - 1:

Flow in both directions
Union - 2 - is closed

B) Union 1 - 2:

Flow in both directions
Union - 3 - is closed

C) Union 1 - 2:

Flow in only one direction
Union - 3 - is closed
Valve must close when a vacuum is applied to union - 1 -