Help tracking down vacuum leak on a 1955 Series 62 CdV

[craig-o]

After reviewing the service manual and searching the forum archives, it doesn't look like there's a diagram that shows all of the vacuum lines for a '55.  If anyone has one, I'd love to see it and be proven wrong...

So I'm hoping for some help to chase down what I suspect is a vacuum leak that is making it hard to accurately tune/adjust my carburetor.  I've posted about some carb issues, and attempted to perform the mixture adjustments per the manual, but I'm unable to adjust the idle using the idle adjustment screw on the base of the Rochester carburetor.  Moving that screw from fully-closed to nearly falling out doesn't change the idle speed at all.  This leads me to believe that I have a vacuum leak somewhere that is bypassing this screw.  The carb was rebuilt and reinstalled recently, and it took 3 tries to get the right manifold mounting gasket, so I'm pretty sure the leak is coming from one of the many vacuum lines.

Here's where I know there are vacuum lines:
Distributor vacuum advance (I recently had this rebuilt and am confident it is holding vacuum)
Power Brake Booster
Windshield wipers
Windshield washer
Radio and power antenna

Am I missing anything?

[Dave Shepherd]

I have used a small propane cannister and opened the valve slightly then pass over the suspect areas, when you find the source the engine will respond. Be careful of course.

[walt chomosh #23510]

Craig,
  Why not cap everything and then see how your idle is?.....walt....tulsa,ok

[craig-o]

Thanks, Walt.  That is my intention, I'm just hoping to see if I've got everything, or if there's some additional line I'm not aware of...

[Jeff Rosansky CLC #28373]

Be sure everything is fully capped off. Dont just trust the device. I thought I had everything capped but there was still vac at the radio. Not sure where it comes from-maybe the little manifold on the firewall? I need to find out for myself but haven't gotten around to it yet. Also don't forget about the vacuum pump. I believe there is some type of check valve in there. I don't know for sure but if that valve is bad and the pump itself is bad it stands to reason you could have a leak from the engine driven pump. I don't know this for sure but it is logical. Do you have the wipers disconnected?
Jeff

[craig-o]

Vacuum pump?  I thought all of these were run from the intake manifold - no?

I capped everything I could find from the hard lines with the exception of the vacuum advance.  I was able to find 3 threaded connectors into the intake manifold, one for the distributor advance, one for the power brake booster, and one for the little manifold on the firewall.  It looks like the wipers, washer, radio and power antenna all from from the little manifold on the firewall.

Once everything was capped, I was able to correctly set the idle and the idle mixture per the manual.  I then reconnected the brake booster only and took her for a test drive.  My idle and hesitation on acceleration seem corrected now.  I guess it's time for some new vacuum hoses all around.

Thanks for the help.

Craig

[The Tassie Devil(le)]

It doesn't take much for a vacuum leak to occur over the life of these cars.

There is so much vacuum hosing that there are lots of places for a leak to occur, especially as the hoses get brittle with age.

Just moving a stiff hose can result in a crack.

Bruce.

[Roger Zimmermann]

Vacuum pump?  I thought all of these were run from the intake manifold - no?

Craig, under the oil pump there is a vacuum pump. Just have a look at your shop manual! the check valve for it is on the block, behind the starter motor. From there, a metal tube is going at the back of the RH cylinder head and then with a rubber hose to the small manifold at the firewall. This manifold is not empty, it has 2 rubber valves inside. After so many years, they are no more good. You can open that manifold and replace the valves (which are indeed a rubber flap). David King is selling that rubber part.
Years ago, I did a report on this forum about this vacuum manifold: http://www.cadillaclasalleclub.org/forum/index.php/topic,108350.0.html

[Jeff Rosansky CLC #28373]

Vacuum pump
It's a Cadlilac baby!! You don't want your antenna going down while  you go up a hill!!
Jeff

[Bob Kielar]

Yep there are three lines that go from the firewall distribution block to the right knob on the radio. One for vacuum, one for up and one for down you activate it by pulling and pushing the knob. I believe the vacuum line from the intake manifold is only for the power brake booster the rest of the vacuum is supplied from the pump.

Keep Cruzin,

Bob Kielar

[Lexi]

Found this apparent mid-fifties vacuum diagram which may help.Clay/Lexi

[Roger Zimmermann]

I believe the vacuum line from the intake manifold is only for the power brake booster the rest of the vacuum is supplied from the pump.

Not quite right, Bob: that line coming from the vacuum pump is attached to the vacuum manifold. There is a second vacuum line at the manifold, coming from the intake manifold, which is logical: at idle, the vacuum pump is not turning quickly enough to deliver a good vacuum; in that circumstance, the the intake manifold is the main vacuum source.