Brake Vacuum using PMO Carbs

[dpetteng]

I have had my 3.2 rebuilt and we are using PMO carbs on it. Does anyone know if PMO's come with vacuum outlets for brake boosters? If not, can I fit them?

[VFR750]

Generally the manifold have bosses that are drilled and tapped for fittings.

I don’t think either Weber or PMO have vacuum ports in the carburetor body.

[Walt Fricke]

Right - the manifolds are, if I remember right, cast the same - right side and left side same casting? Then finished differently. So on each side there is a nice boss with no tapped hole drilled into it, so plenty of meat to drill and tap for a fitting.

What I don't know is whether getting a vacuum out of only one cylinder is adequate. It is possible to get a vacuum out of all six, running those lines to a canister or block you make, with seven fittings, the 7th connecting to your brake vacuum hose. That's one way guys with MAP sensors for EFI even things out for the sensor.

An option would be an electric vacuum pump and a canister up front, with some kind of pressure switching.

Somebody will tell how they did it.

[clutch-monkey]

i used an electric vacuum pump from a GM vehicle because the vacuum was insufficient.

[VFR750]

I run both.

Zenith carburetors have ports that were originally used for an enrichment circuit. Now all six are plumbed to a manifold for vacuum.

But the new cam made very little vacuum at idle.

I installed a Leeds “Bandit” vane style vacuum pump. Installed in the front and use a tee and check valves to run the engine and electric pump in parallel.

Now I have a sufficient braking in all conditions.

[CBA]

Reach out to Richard at PMO and ask him. You have to send a fax to him with your question and he will get back to you. I just purchased a set from him and he specifically asked me if I have power brakes or not which I don't so he set my carbs based on my needs.

[cgarr]

we also used an electric pump with a small tank on our 2.8

[KTL]

The single vacuum source on the left/1-2-3 cyl. manifold worked fine for me in a '79 SC with its original vacuum booster. You need to install a check valve to make it work most effectively.



A commonly available 1/2" hose barb fitting works great to connect the hose from the chassis to the manifold. I use a 1/4-18 NPT tap (use a 7/16 drill bit for the hole prior to tapping) to thread the hole for a common brass plumbing adapter fitting. These are available at your favorite home improvement store

It's strange how some guys have found the vacuum to be insufficient on cars that did not originally have vacuum-assisted power brakes installed from the factory. Two of my friends have had a 1970 911 and 1976 911 racecar and they didn't get much brake assist from the vacuum port hooked up to a factory brake booster mounted in the front trunk. Maybe it was because they had a long length of rubber hose that collapsed under vacuum? Whereas the factory installation is a long section of thick, reinforced hose from the upper area of the engine bay, down along the firewall/rear seat area, into a metal tube at the center tunnel. That metal tube runs the entire length of the tunnel and goes up into the front trunk before another short section of hose is used to connect onto the booster nipple.

The guy with the 1970 had the booster and master cylinder assembly nicely installed in the trunk (he did a G50 trans installation and needed the pedals for the hydraulic clutch) but found that he didn't notice any difference in the brakes. Maybe the used donor vacuum booster he bought was failed? The other guy with the 1976, which is cgarr above, installed a switchable electric vacuum pump and a PVC pipe reservoir in the trunk. Worked great and they also have a vacuum gauge so they always know how much "brake help" is in the reservoir just by looking at the gauge on the dash.

In my own case, I found the vacuum port on the PMO manifold (with the check valve) worked great in my '79 SC racecar. Seems to pull enough vacuum all the time to allow the original brake booster to provide plenty of assist. I never found my brakes to be too much effort (they're larger brakes, along with a 930 master cylinder) and i'm not exactly a big strong dewd- 6' tall, 175 lbs and kinda thin.

[KTL]

Originally Posted by cgarr

we also used an electric pump with a small tank on our 2.8

I hope that's a typo, otherwise you just sunk the ship and revealed why your "just a little 'ol rebuilt 2.7" has quite a bit of POWA?

[cgarr]

Just trying to smoke you out to see if you are still around lol


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[Jeff Alton]

Never found the need to run a pump. We tie each stack into a manifold and get a good vacuum source. And yes, we run some decent cams in these motors. YMMV


Cheers

[VFR750]

I added the pump after an exciting event last summer.

A long down hill run, off throttle, light brakes hear and there. Then a truck pulled out. And I stopped, but the booster vacuum had been used up and the pedal pressure went way up for a hard stop.

I just felt I did not need to have casual drives in the woods become a unpredictable braking event.

Track was never a problem.

[boyt911sc]

Originally Posted by Jeff Alton

Never found the need to run a pump. We tie each stack into a manifold and get a good vacuum source. And yes, we run some decent cams in these motors. YMMV


Cheers

The stacks did not come with vacuum ports so we decided to install brass fitting below the butterfly valve as shown in the picture below. Then connected the six (6) vacuum lines to a collector or block. The brake booster had sufficient vacuum supply it would ever need.



Tony