Piper PA-28-161 Pilot Operating Handbook page 141

SECTION 7 PIPER AIRCRAFT CORPORATION
DESCRIPTION & OPERATION PA-28-161, WARRIOR
II
gauge or collapsed line. In the event of any gauge variation from the norm,
the pilot should have a mechanic check the system to prevent possible
damage to the system components or eventual failure of the system.
A vacuum regulator is provided in the system to protect the gyros. The
valve is set so the normal vacuum reads 4. 8 to 5.1 inches of mercury, a setting
which provides sufficient vacuum to operate all the gyros at their rated RPM.
Higher settings will damage the gyros and with a low setting the gyros will be
unreliable. The regulator is located behind the instrument panel. Vacuum
pressure, even though set correctly, can read lower at very high altitude
(above 12,000 ft), and at low engine RPM (usually on approach or during
training maneuvers). This is normal and should not be considered a
malfunction.
REPORT: VB-1180
7-14
ISSUED: AUGUST 13, 1982