Intake Manifold Vacuum T-Fitting

[Mazlures]

Good afternoon guys. Does anyone have a picture of the stock (or one that would work) intake manifold vacuum T-fitting that would go on my 1972 GMC Sierra Grande!? I has a 350 sbc on an automatic TH400, I believe it is a 3 way prong fitting but I’m not quite certain of what size the prongs need to be. I believe it goes like this:

Brake Booster - 3/8” hose size
Transmission Modulator - !? Size
AC - !? Size
Thread size to bolt onto intake manifold - 3/8”!?

I would really appreciate some guidance on what fitting would work and where I could get one please!!

Thank you very much in advance, have a blessed day.

[PbFut]

The 3/8" to brake booster hose should have a 3/8 fitting at the base of the carburetor both front and/or back. Back to the Brake booster. Front to the PVC valve. The 3/8 NPT Fitting that threads into normally the #8 port is a readily available part. Depending on the accessory list you have, the fitting will have 1 to 3 barbs. 1/4" barbs. 1 to AC reservoir, 1 to transmission, the 3rd, I suspect was to the vacuum gauge or possible cruise control but it is rare to see a 3 barb fitting. My truck only had 2 ports and did not have a vacuum gauge. If you add anything needing a vacuum, Avoid Teeing from critical operational devices or the PVC line if added item bleeds. The carburetor should also have a couple small line connections. One to the distributor vacuum canister. That barb goes into the venturi just on the atmosphere side of the throttle plate. Crack the throttle open and the port opening is now seeing engine vacuum. So at idle the distributor does not see vacuum. Open the throttle and distributor sees vacuum at a level relative to engine load as vacuum is affected by load. Many tuners like full vacuum at the distributor and change the connection point. That is an argument for a different thread.
Carb probably has one or two other small barbs, 1/8" that give full time vacuum. Those are good for the gauge. If you need vacuum that is not engine timing related, tee from the AC reservoir AC side. If you need engine vac timing, best to use the carb port not affected by the throttle plate, or second best is change the bib on the manifold port. Last is Teeing to existing line but you risk diluting the vacuum level of whatever that particular line is feeding.
Sorry dang pic flipped. 1st one is front of carb. Small line is distributor, throttle plate affected. Big one is PVC. Capped port is full time vac.
Second photo shows factory hard line that goes towards brake booster and the 2 barb port.
Maybe someone can flip these photos.

[cericd]

Typing words to allow posting of rotated images...

[Mazlures]

Thank you very much PbFut for such thorough explanation. This will definitely help me buy and make the right connections. Also, thank you cericd for flipping the photos!!👍

[custom10nut]

I searched high and low for a “Two Port” fitting, as I need it for the Trans and Brake vacuum.
If I remember right I found it in a salvage yard, on a Ford truck manifold (I know, don’t hate me, but it works).

[Getter-Done]

Originally Posted by custom10nut

I searched high and low for a “Two Port” fitting, as I need it for the Trans and Brake vacuum.
If I remember right I found it in a salvage yard, on a Ford truck manifold (I know, don’t hate me, but it works).

Yes

Your exactly right. I was raised with both Brands.

That fitting will work.

[Rust_never_sleeps]

When I added a brake booster, I got a fitting from these guys(found ref on the forum):
https://www.corvettecentral.com/sear...vacuum+fitting