Power brakes on Carb 911

[proporsche]

I am doing this engine 1975 911S which is in 1978 body.
No power brakes so you basically breaking with your eyes.
I was thinking since all is in the car for the P brakes just needs to be connected to air.But.
Is it possible to put a fitting on the intake manifold with Weber carbs?
Is is not going to loose too much air?
thanx for any advice

Ivan

[john walker's workshop]

Search vacuum pump.

[proporsche]

Thanx John i did hear something about it..i will look

Ivan

[911 SLANT]

I run power brakes with my 1971 2.8 with PMOs. Tapped into intake manifold with a one way air valve supplied from Richard at PMO

[frankc]

As @911 Slant said, the PMO manifolds have a boss on cylinder 2 for this purpose (circled in photo below).

[proporsche]

Originally Posted by frankc

As @911 Slant said, the PMO manifolds have a boss on cylinder 2 for this purpose (circled in photo below).

thanx, that is what i have ..now, where would i look for the one way valve besides getting it from USA?

Ivan

[Locker537]

There is a one way valve on the booster as well. I'm not sure when it was added, but it's there on my SC and likely Carrera 3.2 as well.

I also have a single port off the driver's side intake manifold for vacuum to the booster. I have seen others with a vacuum manifold to tie into both bank's intakes. They are marketed for EFI ITB cars.

[nickelplated5s]

ProP - what's the gauge on the bulkhead? Between the shock towers.

[john walker's workshop]

In my experience using vacuum from one port, rather than all six results in assist on the first couple of pumps, but the assist fades the more you continuously pump, like going down a steep hill.

[proporsche]

i`ll take your advice John -looking for an air pump ..but i will try also the no 2 port fitting -thinking using no 5 as well since there is a place for the fitting there too.
Anything would be better than this brake system...you almost have to go to the gym before trying to brake the 911;-)

Ivan

[john walker's workshop]

I have a 2.0E carbed engine in my '71 vw single cab pickup, using a vacuum pump and it works great. It runs constantly, so might upgrade to one that shuts off at a certain point when it pulls the amount it was designed for. Thinking longevity.

[frankc]

Originally Posted by nickelplated5s

ProP - what's the gauge on the bulkhead? Between the shock towers.

Fuel Pressure gauge sitting on top of PMO pressure regulator (see photo below for better view).

@proporsche,
The photo below shows where I have the vacuum check valve located (circled).

John certainly knows his stuff, but I will add that I've been driving the car with this setup for 37 years, and have have not had an issue with losing brake boost yet. Perhaps the long run of tubing from booster to engine provides functionality similar to a vacuum canister - as long as the check valves are doing their job. But a dedicated vacuum pump would seem to be an improvement since vacuum is being pulled from just one cylinder (what is the failure rate on these pumps?).

[thetorch]

Seems like an electric vacuum pump would be a lot easier and possibly safer, or do both?

https://forums.pelicanparts.com/porsche-911-technical-forum/1107512-electric-vacuum-pump-power-brakes-carbs.html

[nickelplated5s]

Thanks Frank. Missed your user name on the quoted photo. Nice setup, might switch to those filters.

[proporsche]

Originally Posted by frankc

Fuel Pressure gauge sitting on top of PMO pressure regulator (see photo below for better view).

@proporsche,
The photo below shows where I have the vacuum check valve located (circled).

Thanx Frank..btw nice beast;-)

Ivan

[Locker537]

I do agree the boost is inconsistent using one port off one intake. I can always brake, but the feel and modulation differs. This is a track car.

I like the vacuum pump solution.

[jwetering]

I drilled and tapped the intake manifold and installed a brass compression fitting then used 1/4" copper tubing to provide vacuum to the booster. Worked really well and looked good.

I tapped into 4 of the six intake runners. This was a while ago so no photos - sorry.

[pampadori]

My street car (not a lot of two-foot driving where both throttle and brake are used at same time) doesn't have any issues running boost from only cyl 2. I can only notice it if i'm coasting in neutral and do repeated brake depresses. If i leave in gear and allow engine to pull greater vacuum it isn't noticeable.
This is with a recently rebuilt booster. Prior to that it was more of an issue.

[LJ851]

I’m sure camshaft overlap plays a big role in how well this setup works. My 3.2 Carrera engine with stock cams has plenty of vacuum from just one cylinder to run the power brakes. When my original booster was not operating properly this was not the case.

[proporsche]

Originally Posted by jwetering

I drilled and tapped the intake manifold and installed a brass compression fitting then used 1/4" copper tubing to provide vacuum to the booster. Worked really well and looked good.

I tapped into 4 of the six intake runners. This was a while ago so no photos - sorry.

just about to work on it ,i also like the 4x..next is to find the correct fittings
thanx to all for the ideas..

Ivan