Brake vacuum
 

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.

Re: Brake vacuum
 

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.

Re: Brake vacuum
 

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.

Re: Brake vacuum
 

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

Re: Brake vacuum
 

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?

Re: Brake vacuum
 

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?

Re: Brake vacuum
 

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.

Re: Brake vacuum
 

How much vacuum do you have at 1000 rpm?

Re: Brake vacuum
 

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

Re: Brake vacuum
 

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? :lol: My dad used to get so ticked off if one of us burned rubber in the driveway or the street. :smoke:

Re: Brake vacuum
 

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

Re: Brake vacuum
 

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.

Re: Brake vacuum
 

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

Re: Brake vacuum
 

This is the motor that I am running.

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

Re: Brake vacuum
 

Re: Brake vacuum
 

"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.

Re: Brake vacuum
 

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)?

Re: Brake vacuum
 

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.

Re: Brake vacuum
 

Thanks for clarifying.

Re: Brake vacuum
 

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.

Re: Brake vacuum
 

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.

Re: Brake vacuum
 

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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Re: Brake vacuum
 

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.

Re: Brake vacuum
 

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

Re: Brake vacuum
 

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

Re: Brake vacuum
 

Backing down the driveway in neutral is in your future.

Re: Brake vacuum
 

:waah::lol:

Re: Brake vacuum
 

Delete the booster, use a smaller bore master cylinder (1" or 7/8") for more line pressure, move the master cylinder pushrod to the upper hole for more leverage on the master cylinder, swap to a semi-metallic pad that has a higher coefficient of friction.


The "need" for boosted brakes is one of the biggest wives tales in the C10/classic car world. Manual brakes can be more than strong enough with the simple things I listed.


When you swap from manual to power brakes, do people realize that a lot of what is involved with swapping to power brakes actually takes away the leverage that your foot would normally convert into braking power? Examples-

The stock master cylinder is 1" but the typical boosted system uses a 1.25" M/C. Basic hydraulic principles tell us that a larger bore will make less pressure at the caliper/wheel cylinder than the same amount of pedal pressure would make with a smaller stock 1" bore.

The pedal ratio is changed when you move the pushrod to a lower hole on the brake pedal- leverage that could be making more line pressure is taken away when you lower the mounting point.

Some brake kits, and all parts stores will tell you that ceramic pads are an "upgrade". They are, but only in terms of lifespan and amount of dust they make. Ceramic pads actually have the lowest coefficient of friction of all normal pad types. That requires you to press the pedal harder to create the same amount of friction between the pad and disc. Pad compound can make a huge difference in how well the brakes work; the last car I bought had fairly terrible braking performance- borderline unsafe. Everything was 100% stock and in good condition so it should've stopped just as well as my other identical car. I deglazed the rotors and swapped out the unknown pads for semi-metallic pads, did the procedure to bed-in the pads (this makes a big difference just by itself), and the brakes went back to performing like normal.


So... you take away significant braking power in 2-3 ways depending on what pad type you pick, than add braking power back with a booster. Makes a lot of sense... when you're a vendor selling parts.


Sticking with a small bore master cylinder for more line pressure (simple hydraulic principle at work there), leaving the pushrod in the top hole so that more of your applied pedal pressure makes it into the master cylinder, and using brake pads that are actually good at making friction (semi-metallic) will go a long way in making the truck stop really well without excessive pedal pressure. The pedal will also be more responsive and have better feel, boosted brakes usually feel fairly vague or are overly sensitive to pedal pressure. And if you still need more braking power or prefer less pedal pressure, you can drop down the master cylinder bore size for more line pressure.


My own C10 has a 1" bore dual circuit master cylinder to convert it to a split system, the stock/upper pushrod hole is used, stock rear drums, and the front has been converted to disc using CPP's standard drop spindle/disc kit that uses plain squarebody rotors and calipers. The pads are semi metallic. Nothing fancy. The pedal feel is excellent and pedal effort is normal. The brakes are easily powerful enough to make full use of the tire's grip well above any legal speed without needing excessive pedal pressure. Since the pedal is responsive I can modulate the pedal and hold the brakes at the limit of tire grip right before lock up in an emergency stop situation- something that is harder to do with less responsive boosted brakes.

Re: Brake vacuum
 

That seems so counter intuitive in the hydraulics world because Area X Pressure = Force.

Re: Brake vacuum
 

That is true at the caliper (output force), but not the master cylinder (input force).

https://www.joesracing.com/master-cylinder-math/

Re: Brake vacuum
 

Yes. A smaller master cylinder piston raises pressure IN the system. A larger caliper piston raises force OUTSIDE the system. You can, of course, do both.

Re: Brake vacuum
 

I run a manual disc/drum set up. It was easy. I bought a '71 manual disc/drum MC from the parts store and bolted it up. My brake pedal only has one hole for the push rod and I used it. My proportioning valve is from a squarebody.
It all works fine and takes no more effort to stop than any other manual brake vehicle, in fact pedal effort feels stock except I get the increased stopping power with less fade from the discs....and no heavy unactractive booster to stick my MC out over my valve cover.

Re: Brake vacuum
 

I have an 11" brake booster on my 65 Suburban 283. Every time I stepped on the brakes the booster stole all of my vacuum (15"). the booster wanted 18", so bad brakes and vacuum advance swung all over the place resulting in a bad running engine. installed a separate vacuum pump for the booster and now everything works fine. Vacuum for the timing is manifold vacuum. No other vacuum demands except PCV

Re: Brake vacuum
 

Im currently running the same engine at 5280 ft. I couldn't get more than 11 out of it when running a carb and ended up adding a pump.

Re: Brake vacuum
 

:whine:

How much does elevation make a difference. I am at around 607’

Re: Brake vacuum
 

Elevation variation will yield less boost when the atmospheric pressure is lower, so at your elevation, you're pretty much getting the full benefit of that. Something else must be going on with your braking system.

Re: Brake vacuum
 

Just want to throw this into the mix here.
I have had issues with brakes and vacuum on two different engines, my "69 C10 with 5.3 LS - it has maybe 10" to 11" of vacuum at idle (note Idle is set to 800-900rpm or so). I have tried every possible idea for vacuum brake booster I could imagine, size of booster, different size bore masters (note I am running Wilwood brakes and master) even tried a vacuum cannister and then out of frustration I bought a damn electric Vacuum pump.. and well the noise from that alone was not working for me... anyways, I spoke with Wilwood and basically came to the understanding that they require a min. of 15" but prefer 17" of vacuum. So, I was told to consider a Hydro-boost brake system. Needless to say, I got one and DAMN what a difference! It is not cheap, but works great!
So on my "66 I am running a 383 stroker with a cam that produces maybe 10" if lucky, so I have already purchased a full Hydro-boost system for it, I am convinced they are they are great.
Now just to add 1 more idea, I have learned about an electric stand alone Brake system that is now offered. Not sure on these, never tried it but those that have love it, but be ready to pay for it.
Anyways, just wanted to toss these ideas into the mix also. I am not sure but as some have mentioned they have great luck with min. vacuum but I am unlucky in this. LOL Wish you luck!!

Re: Brake vacuum
 

What brand of Hydro-boost did you get?

Re: Brake vacuum
 

There are solenoid-assist brake systems in some of the newer cars. Old tech, in a way, like and electric rack and pinion. If I had this much trouble, I'd go to a manual system. It's always disconcerting when the engine stalls. My wife was driving our Astrovan (long gone) when the fuel pump quit. She managed to coast off the freeway and off an offramp with no hydra-boost or power steering.

Re: Brake vacuum
 

I have a bad knee. Would switching back to manual breaks require more force than hydroboost breaks?

Also when I had my truck painted and my firewall shaved I had a captain fab bracket installed and I have no idea what things look like underneath it and what was done with my original bolt holes.