Ford 2003 Mustang Workshop Manual page 3058

Disconnect: Vacuum Supply Hose.
Connect one end of the vacuum supply hose to a vacuum pump
and test for leaks.
Does the hose leak?
A4 CHECK THE VACUUM CHECK VALVE
Key in START position.
Check the A/C vacuum check valve for correct installation by
removing (one at a time) the reservoir hose and the control
assembly source hose from the A/C vacuum check valve. Check
for vacuum.
Is vacuum available at both check valve ports?
A5 INSPECT THE VACUUM CHECK VALVE
Key in OFF position.
Disconnect: A/C Vacuum Check Valve.
Inspect the A/C vacuum check valve for an obstruction and
correct operation. Airflow through the A/C vacuum check valve
should be in the direction toward the engine (engine connection
site is marked "VAC").
Is the A/C vacuum check valve plugged or obstructed?
A6 LEAK TEST THE VACUUM CHECK VALVE
Key in OFF position.
Disconnect: Vacuum Check Valve.
Connect the A/C vacuum check valve to a vacuum pump with
two hoses and a tee fitting.
Yes
REPAIR or INSTALL a
new vacuum hose.
TEST the system for
normal operation.
No
GO to A4 .
Yes
GO to A6 .
No
GO to A5 .
Yes
INSTALL a new A/C
vacuum check valve.
TEST the system for
normal operation.
No
GO to A6 .
Yes
INSTALL a new A/C
vacuum check valve.
TEST the system for
normal operation.
No
RECONNECT the A/C
vacuum check valve.
GO to A7 .