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

[Mazlures]

Oh I’m with you guys, I don’t care where the part comes from, I just wanted to work!

Thank you for the website, I kid you not, I found it scrolling on the internet just a few hours before you sent it. They have just about every fitting, 1,2 or 3 prongs!!haha

I have another question, I have seen a few trucks at different events such as C10 Nationals, GoodGuys, etc. that have it set up with a 3 Prong Fitting at the intake manifold, one for the Brake Booster, one for the Tranny and one for the AC and they plug the back of the carburetor.

My question is, is that a good set up as far as having good vacuum!? Or is it better to put your Booster at the rear port of the carburetor and use a 2 Prong Fitting at the intake manifold for only the Tranny and AC!?!

I’ll try to attach some pictures of what I have seen.

[72c20customcamper]

I really wouldn’t want my power vac hooked up with any other vacuum lines . It should be by itself .

[Mazlures]

So booster to back of the carb and tranny and ac to the intake, got it! Thank you all very much for the advice!

[PbFut]

100% agree. More a safety thing. You have 2 or 3 times the risk of a hose coming loose and killing, or at least reducing, the vacuum in the head of the fitting and up to the booster. Having the booster by it's self at the carb flange will insure the best most reliable source. As far as performance, not much difference as the tranny and AC don't bleed vacuum once charged. Now if it were an Idle Control Motor for a Fuel Injection system, then you would really have a problem with your brakes. They would not play well together.

[Mazlures]

Oh okay perfect, thank you guys a lot!! I’m not a mechanic by any means but I know a tiny bit from always helping my dad work on his own trucks while I was growing up. My dad, y’all, YouTube, and google is what has kept me in the game while trying to restore my truck.

I bought it about 4 years ago and it was unbelievably bad under the hood, missing all sorts of things, wire harness was just spit out from the firewall, wrong things in the wrong places, all the way form the rear end up to the engine, you name it… This truck has not ran in who knows how long!! But I love it so I’m working hard on it every time I get a chance.

Here is the before and how I have it so far right now:

[Mazlures]

Here are some pics.

[Mazlures]

Some more.

[Mazlures]

Here is the truck back when I first bought it. I don’t have a recent picture of it.

[PbFut]

Good to see you are enjoying the journey.
A really good concept to keep in mind when doing things that affect air flow to the engine... The engine is best viewed as a big a$$ air pump. So, anything you do to affect the air flow at any point, from in to out, will affect the performance of the engine. Some, so minor it's not noticeable. Other changes will render the engine unable to run. Everything you do is a balance of compromises, balancing the results, hopefully to the net positive overall.
Last point. Air flows the same as water except water cannot be compressed like air. It is easier to understand flow in water because you and see and better feel it. But other than vacuum and compression, the flow is identical. Keep this in mind from air cleaner to muffler and it is easier to visualize intended modification affects.
I am playing with a down draft ITB system from Speedmaster right now. It is the worst designed kit I have ever experienced from the aftermarket. Incorrect or even missing port provisions for most of the EFI sensors. So all this is keenly in the front of my brain right now as I obsess over the details to make this kit remotely drivable. If I get it right it will be a really cool looking system. If I fail, an expensive piece of garage wall art.

[Rust_never_sleeps]

If I had a port on the carb, I'd definitely use it, but when I searched this topic, plenty of folks are using manifold fittings.

IIUC, if any of those other hoses break, the diminished vacuum will be constant across the manifold anyway(speed of sound), and the check valve in the booster is the only thing that will help

Personally, I have no problem stopping my truck without boost, I just don't enjoy doing it over and over if I don't have to

[Mazlures]

Oh man, that sounds too complicated for me!!haha but good luck with all that and we’ll be praying it comes out right so it’s not just a waste of money. Definitely post some pictures though, it sounds cool!

Well once again, thank you very much for all the help and advice, I truly appreciate all the knowledge you guys pitch in.

I often feel like I need some of y’all on speed dial so when I come to a stop, I can make a quick phone call instead of having to create a post and wait.haha

I live 30 minutes from the closest parts store, so it’s a freaking hour trip at least just to go get something. Plus the time I have to wait to figure out what it is that I need…lol

[Mazlures]

Rust-never -sleeps I am definitely not doing a nut and bolt restoration but I do like to keep it as original as possible. Therefore I bought an old Quadrajet (the original to the truck), restored it and since that one does have the back port, I will be connecting the booster to it and putting a T-fitting on the manifold for the other two things.

Pardon my lack of knowledge but what is IIUC?

[Rust_never_sleeps]

Originally Posted by Mazlures

Rust-never -sleeps I am definitely not doing a nut and bolt restoration but I do like to keep it as original as possible. Therefore I bought an old Quadrajet (the original to the truck), restored it and since that one does have the back port, I will be connecting the booster to it and putting a T-fitting on the manifold for the other two things.

Pardon my lack of knowledge but what is IIUC?

Yeah, whatever's clever. There's even a section in Cliff's book about adding a brake port, so maybe next time the carb is off, I'll do that.

IIUC="If I understand correctly".....which really means "I may be full of cr@p"
My tech-speak betrays me at times

PbFut, is there a check valve in the vac reservoir for the AC vent controls? I don't think mine retains vacuum

[Rust_never_sleeps]

Stock fitting looked like this, ICAC(In case anyone cares)
Transmission and AC vent ports, manual drums

[Mazlures]

Awesome, thanks for the pictures and the acronym definitions!!haha

[PbFut]

Originally Posted by Rust_never_sleeps

PbFut, is there a check valve in the vac reservoir for the AC vent controls? I don't think mine retains vacuum

The check valve in the ac reservoir and 2nd one in the brake booster is there to stop the vacuum from bleeding off after engine is shut off. Should alow vents or brakes to function 1 or 2 cycles if engine is off or stalled. The check valve will not protect from a major leak.

[Rust_never_sleeps]

Originally Posted by PbFut

The check valve in the ac reservoir and 2nd one in the brake booster is there to stop the vacuum from bleeding off after engine is shut off. Should alow vents or brakes to function 1 or 2 cycles if engine is off or stalled. The check valve will not protect from a major leak.

Gotcha, totally missed the reservoir function, thanks

some interesting commentary here ICYGAS ;-)

https://67-72chevytrucks.com/vboard/...31&postcount=6