Low vacuum at idle

The service manual for my 1968 Cessna 320F says the vacuum regulators
should be set with the engines at 1700 RPM. If you want to set the right
regulator it says to idle the left engine and set power on the right
engine to 1700 RPM. At this power setting you should have 5" of suction
on the vacuum gage. After doing this for both sides it says to run both
engines up to 1700 RPM and you should get 5" +-.25".

I am getting 5" on each side at 1700 RPM per the service manual, so it
would appear all is OK, however my suction readings are not passing the
limits prescribed in POH/Owner's Manual supplement D53317-1-13. This
supplement says after starting the first engine, with throttle set at 1000
RPM the suction gage should read in the green arc or 4.50" to 5.25". I'm
getting about 2" of suction, but the failure button is sucked in. This is
the same for each engine. So my failure buttons are working correctly
according to the supplements procedures, but I am no where near being in
the green arc, with only one engine running at 1000 RPM.

With one or both engines running at least 1700 RPM I get 5" of suction
and with both engines running at 1000 RPM I get 3.5" of suction, but with
only one engine running at 1000 RPM I get around 2" of suction. Each
failure button is sucked in with the engines running and if only one
engine is running only one failure button is sucked in.

The vacuum pumps are Airborne dry pumps p/n 242CW. I just had the vacuum
manifold p/n 1H5-1 replaced which didn't fix this problem. Why would my
suction be OK at 1700 RPM and above, but be low at low RPM? The
regulators are p/n 2H3-8 and -9. Both regulators are probably the same
age and time on them, is it possible both are bad or is it more likely a
leak in one of the hoses between the manifold and instruments? Is there a
way to pressurize the system to check for leaks? How can this best be
accomplished?

"Chad Lemmen" wrote
With one or both engines running at least 1700 RPM I get 5" of suction
and with both engines running at 1000 RPM I get 3.5" of suction, but with
only one engine running at 1000 RPM I get around 2" of suction.

First thing to check is that both regulators are closing completely at low
rpm.
Simple way to check is to temporarily cover their relief intake screens with
duct tape. If this increases vacuum at idle, that's the answer.
If the regulators haven't been disassembled and cleaned in years, then
accumulated dust/gunk around the regulator's valve seat can prevent it from
sealing shut. Remove the regulator, tap the wrist pin out to disassemble,
give it a good scrubbing in Stoddard solvent, dry, reassemble and reinstall.
Valve should then close and seal completely at idle.

Second thing is to check all vac hoses and fittings for cracks. Rubber
hoses have a nominal 10 year life. Nylon fittings get brittle with age, and
become prone to cracking after perhaps 20 years.