Ford 2003 Mustang Workshop Manual page 3059

Pump 51 kPa (15 in-Hg) vacuum on the A/C vacuum check valve
and observe the gauge reading.
If the vacuum loss exceeds 3.37 kPa (1 in-Hg) per minute,
remove the A/C vacuum check valve from the tester and plug the
vacuum hoses. Pump 51 kPa (15 in-Hg) of vacuum with the
tester to be certain the hoses and tester are not the cause of the
leak.
Does the A/C vacuum check valve lose more than 3.37 kPa
(1 in-Hg) of vacuum in one minute?
A7 CHECK THE VACUUM RESERVOIR
Disconnect: A/C Vacuum Reservoir Tank.
Use a vacuum pump to leak test the A/C vacuum reservoir tank.
Does the reservoir leak?
A8 CHECK THE SUPPLY HOSE
Disconnect: Function Selector Switch Supply Hose.
Use a vacuum pump to leak test the function selector switch
supply hose.
Does the function selector switch supply hose leak?
A9 CHECK THE CONTROL ASSEMBLY
Disconnect: In-line Vacuum Harness.
Connect a vacuum pump to the black hose and plug the other
hoses.
Yes
INSTALL a new A/C
vacuum reservoir tank.
TEST the system for
normal operation.
No
RECONNECT the A/C
vacuum reservoir tank.
GO to A8 .
Yes
REPAIR or INSTALL a
new function selector
switch supply hose.
TEST the system for
normal operation.
No
RECONNECT the
function selector switch
supply hose. GO to
A9 .
Yes
NOTE the function
selector switch position
where the vacuum
drops. GO to A10 .