Low vacuum for brakes.

Gman1987

Gman1987

Not-quite-so-new-guy
Jan 6, 2019
28
12
3
Upstate SC
#1
Hello to all. I checked the vacuum for brakes and it is around 10 at idle, increases as should as rpm increases. Told this is because of cam in engine. I have researched and seen one solution as running vacuum to a canister first then to brake booster, not sure how this would increase the vacuum to 25. Another is to use a vacuum pump and I see there are suppose to be some quiet ones out now. Looking for suggestions and recommendations. Also, I am not a mechanic and not sure why but when I disconnected the vacuum gauge the engine rpm increased. Is the engine starving for air?
 
fleming442

fleming442

Geezer
Dec 26, 2013
5,466
6,050
113
#2
Vacuum should decrease as rpm rises. 10 is pretty low; unless it is a radical cam. The engine rpm increased because you made a vacuum leak when you unhooked the gauge. Double check that all your ports are capped because a leak will make the reading low.
 
B

bracketchev1221

Master Mechanic
Jan 18, 2018
367
330
43
#3
The quickest fix is the vacuum can. Through a little tuning, you SHOULD be able to have the idle vacuum come up. I was able to get mine up from 7 to 10" vacuum on my Chevelle. But you will see a spike in vacuum after you rev the engine because the throttle blades are closed and the engine rpm is still up. This increase is what vacuum will be held in the can and booster. So even though you may see 10" at idle, the can may be holding 13-14". And the extra capacity of vacuum will allow you a few more pedal pushes before the pedal becomes hard from lack of vacuum.

But with that being said, are you having any brake issues or did you just check it and find the vacuum low. If you don't have a hard pedal, you don't really have a vacuum issue.
 
69hurstolds

69hurstolds

Royal Smart Person
Jan 2, 2006
1,551
2,060
113
#4
Vacuum can just gives you a larger volume of vacuum source due to the check valve on the canister plus the check valve on the booster. Depends on the size of your vacuum canister. It's not a panacea. As bracketchev1221 stated, it's really only if your brake system is totally up to snuff and advised if it's your last resort. If you have a check valve/booster problem, vacuum leaks, or anything else, the vacuum can is just a band-aid. Summit has them for less than $40 unless you go name brand, but I figure they're pretty much all the same.
 
Gman1987

Gman1987

Not-quite-so-new-guy
Jan 6, 2019
28
12
3
Upstate SC
#5
Thanks for all the info. Pedal is hard at idle. I may try a canister to see if it helps before going the route of a pump. Thanks again, good luck.
 
airboatgreg

airboatgreg

Royal Smart Person
Oct 2, 2016
1,466
1,351
113
#6
I am thinking a pump will be the safest way to go.
 
melloelky

melloelky

G-Body Guru
Oct 22, 2017
941
1,413
93
mass
#7
more times than not the canister isn't a good fix if the motor isn't making much to begin with.
 
bob64

bob64

G-Body Guru
Mar 30, 2017
701
629
63
Niagara Falls, Canada
#8
You need 14 lbs of vaccum at idle with no leaks. Get some WD-40 spray it around places you think maybe leaking, and if there is a leak car will idle up.

Before buying anything check the timing. Just acouple degrees can change your vacuum. 2) Make sure your carb is dialed in, which means (to me) that see by adjusting low idle jets where it makes the most vacuum. If both jets are not balanced out it will have low vacuum.

Yes taking the vacuum off the distributer will change the vacuum, its normal.
 
Gman1987

Gman1987

Not-quite-so-new-guy
Jan 6, 2019
28
12
3
Upstate SC
#9
Thanks for all the info. I understand there are pumps out there that are quieter. Any suggestions as to which ones I should stay away from or the ones I should look at. Thanks again for the suggestions and advice, greatly appreciated.
 
B

bracketchev1221

Master Mechanic
Jan 18, 2018
367
330
43
#10
What can you tell me about the engine? I first would want to know how radical or not this engine is before deciding on where the fix should be. At idle the car should at least have a softer pedal. That is the best vacuum the car will develop.
 

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