Thread: Vacuum for Power Brakes!

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I am needing some suggestions on sorting out my power brakes!
Most of the stuff is from Speedway, 4 wheel GM Metric disc brakes, rears have
mechanical parking brake. Single diaphram booster with Corvette style master
cylinder.
The engine is a small block Chevy 383 stroker with a vintage Babe Erson cam
that I have some info on. The duration, 218 degrees @ .050, 104 degree lobe separation, lift .456 Int, .449 Exh.
Pretty lumpy @ Idle.
Intake manifold is Edelbrock Performer EPS with Edelbrock 650 cfm #1406.
I wonder about where to pull the vacuum for the brake booster?
Right now the vacuum is coming from the top right side of the intake manifold.
I know that the carb has a large vacuum port in the back-middle above the base
which is now plugged.
What about vacuum pumps or canisters?
The brakes willl stop the truck but just not good enough to lock them up or stop fast!
Help me out!
Thanks! Joe

How much vacuum is the engine making? What size rotors and calipers? Do you know the bore size of the master cyl?

The engine is making 10 in. of vacuum, the front rotorsare 11 in. rears are 11.75, I don't remember what size bore the GM Metric calipers are, The MC bore is 1 1/8".

You are going to need a auxiliary vac pump with that cam, lumpy cams and low vac are quite common. Summit has a good selection, but I suggest you go to the Master Power brake site. Look at that pump and try to buy that one, MPB's price is on the high side, however it can be found @ a lower price, so look around.

The reason I like this part are two fold, it is completely self contained and will cut off when a vacuum has been pulled, cycles like a compressor, easy install one wire and a hose to the booster... done. Another suggestion would be mount it under the car, these pump make noise like an electric fuel pump. Get 4 rubber mounts, that are used to mount small compressors, like in a window AC, or refrigerators.

The reason a canister will not work is because the canister will only hold the total vac pulled, in other words it does not build up more then what is pulled. The hard brakes are happening because of the 10 in. of vac the booster needs 15 -18 to work, using a canister will still only give you 10 in, of pressure. These pumps can be found in modern cars, motorhomes, they are more common then I thought, but the prices seem to be about the same and all seem to make noise. I did see one that advertised a DB level but the price was off the planet.

Good luck and let us know what you end up doing, fyi my vac 7-8 @ idle and have been using this pump 12 years.

One more question.. what is the distance from the pivot of the brake pedal mounting to the connection of the master cylinder rod and what is the total length of the pedal lever? From the center of the pad to the center of the pivot mount..

The 10 inch of vacuum is a thorn for sure but maybe you can change the master cylinder rod connection and "pick-up" some mechanical advantage / leverage and it will only cost you some labor. If you have the room, you might also add length to the brake pedal..??..?? Even an inch will add to or increase "clamping pressure" of the calipers.

The effect of 10 in of vacuum is most noticeable when multiple brake applications are being made. Such as rolling up to a stop light, with each peddle touch the brake peddle becomes harder to apply with less braking until you just about have to stand on the thing to stop. Get the vacuum up or remove the booster and go manual otherwise you are just gambling.

To demonstrate just go out get the car rolling 30mph, slip it into neutral cut the engine, try and stop. All power assist braking cars will respond the same. Oh yes leave enough room to start the motor or use the emergency brake.

On the canister thing, the canister has a one way check it is possibly to get above the 10 in mark. The reason is if you have a vacuum gage you will see that coming off the power the vacuum will go up. BUT quote Summit :These vacuum canisters capture an extra shot of vacuum from your engine for your power brakes" AN sounds like one and thats not going to give you repeated braking with the same force from the brakes. If you just go around the block to car shows, maybe a fit but is you drive your car for the enjoyment then you should consider a pump or revert to manual non assist brakes.

Me I want my brakes to do their job with the same efficiency every time all the time .

Dollars and sense buy pump: 350.00.
Replace with manual MC: 150.00 + tossing the booster ( 7" DUAL DIAPHRAGM POWER BOOSTER $195.00) total for manual conversion 345.00?

Good info, once my engine goes back in this will be very helpfull!

6 to 1 ratio.

Originally Posted by curmudgeon

6 to 1 ratio.

Should work fine



pedalrati.pdf

Originally Posted by curmudgeon

6 to 1 ratio.

Any way (room?) to increase this?

Not knowing what intake your running, I would move the vacuum hose for the brake booster to the connection now plugged under the carb. It probably won't change anything but it may increase the speed the 10 inches is acquired. You may be connected to a single runner now and being under the carb may increase the vacuum pulses. Probably not enough to really notice but it won't hurt to try.

It seems you know the answers to the questions readily so should I/We assume you've been dealing with this for a while?

You have a couple choices I'm thinking.
1. increase the brake pedal ratio as much as you can.
2. replace the booster with a wider unit if possible,
3. install a vacuum pump to increase vacuum "pressure"...

The larger pedal ratio will increase system pressure by using more leverage against the master. The wider diameter booster will apply more force when compared to your current booster. And the vacuum pump will also increase system pressure by increasing the force applied inside the booster. I think options 1 and 3 are the most effective, but not knowing your car, it's a swag for sure.

Do you or have you tested brake system pressures? If you could install a gauge and record system pressures front & rear as a starting point. then when you make a change, you can re-test and verify if your changes are having a positive effect!

Great idea!

I thank all of you for your ideas! The truck is presently at a friend's house having an
oak roof structure built for it. As soon as I get it back I will get after it with a vacuum gauge, relocate my vacuum takeoff at the carb base and go from there.
I will keep you posted as I sort this out and let you know what I find!
Thanks! Joe

"relocate my vacuum takeoff at the carb base and go from there."
Not thinking that will change anything, the vac will be the same. Before you do any plumbing check both ports with a gage.

Have read when you have the time, this will clear the air and explain everthing:
Master Power Brakes - Reference/FAQs: Power Boosters

I' ve been going over my brake system and this is what I have now:
The biggest problem I had was not enough throw from the pedal pivot to the master cylinder rod. I extended the throw but lost some leverage up top! But before I was not moving the MC piston enough. Now If I nail the brakes it will lockup and slide, a vast improvement!
I have found that my engine makes 9" of vac at idle, bring it up to 1200 rpm, it will produce 20". So now I have to decide how I want to go about producing vacuum when I'm in traffic and can't produce the RPMs' to bring up the vacuum.
At least now I know I can stop but sometimes it just takes more pedal pressure!
It's probably time to buy a pump, Summit has a couple self contained units made by CVR
that look like an easy install!

Originally Posted by curmudgeon

I' ve been going over my brake system and this is what I have now:
The biggest problem I had was not enough throw from the pedal pivot to the master cylinder rod. I extended the throw but lost some leverage up top! But before I was not moving the MC piston enough. Now If I nail the brakes it will lockup and slide, a vast improvement!
I have found that my engine makes 9" of vac at idle, bring it up to 1200 rpm, it will produce 20". So now I have to decide how I want to go about producing vacuum when I'm in traffic and can't produce the RPMs' to bring up the vacuum.
At least now I know I can stop but sometimes it just takes more pedal pressure!

Good to hear the news. Thanks for the update. Hope we were helpful.