Brake vacuum

[forestb]

I have front disks, rear drums with power brakes and an automatic transmission I recently installed a new motor with a more lumpy cam. I checked my vacuum at the hose that comes out of the back of the carb and at idle it fluctuates between 10 and 12. When I drive around normal it works fine but my parents house has a vary steep driveway and when I am backing out I am not able to bring it to a full stop until I hit the bottom. Is that enough vacuum. If I remember correctly 12 is a good amount.

[Steeveedee]

You mean at the bottom of the driveway, not the pedal goes to the floor (bottoming), right? Is your foot constantly on the brake pedal, or are you on and off of it? With low vacuum, the engine may not be able to pull it fast enough and you can lose the assist if you are pumping it, even a little.

[forestb]

Should I assume that you feel that 10 to 12 is not enough vacuum?

I meant when I hit the bottom of the driveway. I am holding it constant not pumping it. The break peddle is bottoming out at this point but not on the floor it just hits a hard spot.

[forestb]

So I just saw on the summit website that it should read around 16-18.

[geezer#99]

14hg of vacuum should be enough.
You need to increase your vacuum.
A couple ways to do it.
A vacuum pump or fine tuning your carb and timing.
If you close your throttle completely your vacuum will increase.
Then to get it to idle you’ll need to increase your initial timing.
What carb?
What’s your initial timing at?
Where’s your vac advance connected to?

[Steeveedee]

So I guess your vacuum is too low for that booster. You may have to install a vacuum pump. That must be one rumpty cam. Is the booster single or double diaphragm?

[forestb]

My engine is currently detuned for the breaking in process. I need to go another 200 miles before I can adjust the timing for better performance. It sounds like that might help increase vacuum.

[geezer#99]

How much vacuum do you have at 1000 rpm?

[MT65]

According to the people at Master Power Brakes, 18” at idle is the number you need, can you get by with less…sometimes…..

[Steeveedee]

Originally Posted by geezer#99

How much vacuum do you have at 1000 rpm?

Right? Maybe leave it in neutral and coast down that steep driveway? I've never run a big enough cam that I didn't have vacuum-assisted power brakes work properly. Though now that I think on it a bit, those cars didn't have power brakes.

I know! Spin a doughnut and head down hill going forward! What could go wrong? My dad used to get so ticked off if one of us burned rubber in the driveway or the street.

[geezer#99]

In neutral or just hot foot it.
Hot foot it still gives you control if you need it.

[forestb]

Going down in neutral sounds like a good idea. It should help. It is a little dangerous. I will check to see what the vacuum is at 1000 rpm next weekend. My tools are not in the same place that I live.

[rbruno68]

What motor are you running? What are some of the specs of the cam and motor?

[forestb]

Originally Posted by rbruno68

What motor are you running? What are some of the specs of the cam and motor?

This is the motor that I am running.

https://blueprintengines.com/product...ads-roller-cam

[vin63]

This is the meme I used to send to my friends who insisted I run a vacuum booster on my build, since I'm still thinking of putting a 6-71 supercharger on mine:

In all seriousness, I would probably do away with the vacuum booster if you plan to keep your engine. It's those low speed, in-traffic situations that you'll need the immediate brake response the most. I run a Wilwood MC (no booster) and their 6-piston caliper up front with 4-piston Mark Williams disc brakes out back. Driving in our SoCal traffic, I'm thankful for the instantaneous brake response.

[forestb]

"do away with the vacuum booster if you plan to keep your engine".

Do you mean that using a booster will damage the engine or do you mean that I will have to make the decision between my brakes working (while using a booster) or changing the engine?

I think my next step is to wait and see how much vacuum I end up having after the engine is properly tuned and then start changing things if I am still not getting enough vacuum.

[SkinnyG]

For the record, my '77 C10 idles at 8inHg in drive, and has very acceptable brakes, such that I don't even think about them when driving it, and it's been my daily for the past nine years.

Having said that, 8inHg is LOW, but my brakes are still fine. I would keep the booster, and add a vacuum pump if you simply cannot get the brakes to work adequately.

My '61 has Hydroboost.

Questions, I guess - is the booster broken? Operational? is the one-way check valve working correctly? Are the brake shoes installed correctly (small shoe to the front)?

[vin63]

Not at all. I'm saying that if your engine combination (and you plan on keeping the engine combination) is not making enough vacuum for the booster you have, then I would not run the vacuum-based booster. My current engine combo does not make much vacuum either, so I didn't install a vacuum booster for my brake system. As the others have mentioned, double check that the rest of your braking system is working properly.

Originally Posted by forestb

"do away with the vacuum booster if you plan to keep your engine".

Do you mean that using a booster will damage the engine or do you mean that I will have to make the decision between my brakes working (while using a booster) or changing the engine?

I think my next step is to wait and see how much vacuum I end up having after the engine is properly tuned and then start changing things if I am still not getting enough vacuum.

[forestb]

Originally Posted by vin63

Not at all. I'm saying that if your engine combination (and you plan on keeping the engine combination) is not making enough vacuum for the booster you have, then I would not run the vacuum-based booster. My current engine combo does not make much vacuum either, so I didn't install a vacuum booster for my brake system. As the others have mentioned, double check that the rest of your braking system is working properly.

Thanks for clarifying.

[rbruno68]

I am hoping to run a similar cam in mine with power brakes. My cam isn't a roller. I guess I better consider this before putting the engine together.

[forestb]

Originally Posted by rbruno68

I am hoping to run a similar cam in mine with power brakes. My cam isn't a roller. I guess I better consider this before putting the engine together.

As stated before the engine has not been tuned to it full potential. So I am not sure what the vacuum will be like when it is properly tuned.

[SFFD103]

The 66’ I just bought with a rebuilt 350 has a “Thumper” cam and a dual master cylinder. A vacuum pump was added for the lope.
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[forestb]

My old engine had a pretty big cam and I had a vacuum pump. I didn’t like it so I switched out the cam in order to get rid of the pump. The pump made the brakes work grate I just didn’t like hearing it all the time and also the fear of it breaking down and losing my brakes which happened to me once when the relay stopped working.

[forestb]

I also tried a canister for a while but that didn’t help all that much.

[forestb]

So I measured my vacuum at 1000 rpm and the gauge read 16.