Vacuum Issue
#1
Vacuum Issue
Hi! I'm having a braking issue. Altbough the vacuum gauge reads 15" Brakes are not locking well and the pedal feels slightly hard. A vacuum canister did not improve the issue. I have a 455, Edelbrock performer intake, and a mild camshaft. I was told that between 14 to 20" is fine, but it seems that is not my case. There are no vacuum or fluid leaks. Everything looks good. Any ideas? I was told to install a vacuum pump.
#2
You might want to determine if the booster is working properly before spending any money on an aux vacuum pump. 15-20 in of vacuum is more than enough for a stock booster. We ARE talking about a stock booster, aren't we? This isn't some 7" aftermarket booster, is it?
Has the booster worked previously? When did you loose assist force? The control valve in the booster can stick, preventing it from working properly.
Has the booster worked previously? When did you loose assist force? The control valve in the booster can stick, preventing it from working properly.
#3
You might want to determine if the booster is working properly before spending any money on an aux vacuum pump. 15-20 in of vacuum is more than enough for a stock booster. We ARE talking about a stock booster, aren't we? This isn't some 7" aftermarket booster, is it?
Has the booster worked previously? When did you loose assist force? The control valve in the booster can stick, preventing it from working properly.
Has the booster worked previously? When did you loose assist force? The control valve in the booster can stick, preventing it from working properly.
#5
That is scary. What else could it be? I mean, it just that I dont feel them locking like to stop on a dime. The brake pedal feels slightly stiff but not much.
#6
[QUOTE=Don Roberto;1048676]That is scary. What else could it be? I mean, it just that I dont feel them locking like to stop on a dime. The brake pedal feels slightly stiff although not much.
#7
My friends 66 Chevelle had the same issue with his front disc conversion. The pedal felt a bit stiff but the brakes sucked. 4 mechanics looked at and could not figure it out. I swapped his power booster with a stock replacement combined with additional mods to make the aftermarket master cylinder work with it.
Is the rod connected to the lower hole on your brake pedal?
Is the rod connected to the lower hole on your brake pedal?
#8
My friends 66 Chevelle had the same issue with his front disc conversion. The pedal felt a bit stiff but the brakes sucked. 4 mechanics looked at and could not figure it out. I swapped his power booster with a stock replacement combined with additional mods to make the aftermarket master cylinder work with it.
Is the rod connected to the lower hole on your brake pedal?
Is the rod connected to the lower hole on your brake pedal?
#9
So there are two holes on the brake pedal. The booster's rod was attached to the one closer to the pedal's rubber. So does it mean that the rod needs to be attached to the outermost, top hole?
#10
I didn't say "check valve". I said CONTROL VALVE. There is a valve built into the booster where the pushrod from the pedal enters. This valve senses your pressing on the brake pedal and activates the booster. If the control valve goes bad or sticks, the booster doesn't know you are pressing on the pedal, thus no boost.
If the rod were in the wrong hole, you would have the same pedal ratio as for manual brakes, which would make it EASIER to press on power brakes, not harder. It's the other way around (using the power brake hole with manual brakes) that causes problems.
So the brakes worked fine with the ORIGINAL booster, but don't work now with the aftermarket booster?
HMMM. I WONDER what the problem could be...
If the rod were in the wrong hole, you would have the same pedal ratio as for manual brakes, which would make it EASIER to press on power brakes, not harder. It's the other way around (using the power brake hole with manual brakes) that causes problems.
HMMM. I WONDER what the problem could be...
#11
I didn't say "check valve". I said CONTROL VALVE. There is a valve built into the booster where the pushrod from the pedal enters. This valve senses your pressing on the brake pedal and activates the booster. If the control valve goes bad or sticks, the booster doesn't know you are pressing on the pedal, thus no boost.
If the rod were in the wrong hole, you would have the same pedal ratio as for manual brakes, which would make it EASIER to press on power brakes, not harder. It's the other way around (using the power brake hole with manual brakes) that causes problems.
So the brakes worked fine with the ORIGINAL booster, but don't work now with the aftermarket booster?
HMMM. I WONDER what the problem could be...
If the rod were in the wrong hole, you would have the same pedal ratio as for manual brakes, which would make it EASIER to press on power brakes, not harder. It's the other way around (using the power brake hole with manual brakes) that causes problems.
So the brakes worked fine with the ORIGINAL booster, but don't work now with the aftermarket booster?
HMMM. I WONDER what the problem could be...
"You should have at least 18” vacuum to the booster. Anything lower will begin to give a hard pedal. If the vacuum level is below 18” you may be able to tune the engine and bring the vacuum level up to that level. If the vacuum level is around 16” the addition of a vacuum reserve canister will improve the braking. If the vacuum level is below 16” you will need to add an electric vacuum assist pump to supplement the engine vacuum."
What is your take on this?
Last edited by Don Roberto; October 9th, 2017 at 05:37 PM.
#13
It also has a tendency to bind the rod in the upper hole and damage the bushing in the booster.
#14
#15
Im having the same problem with my aftermarket "original looking" power booster. More than likely its Chinesium, and not likely to be as good as an original. I did try an after market electric pump and reservoir which did not help. So i started jacking with timing and carb adjustments and got my vacuum up to almost 15" which helped some. It was at 12 inches before. It wont build much vacuum just idling but if i get it out and drive it, i have pretty good brakes, it wont quite lock em up but it will stop quick. Almost like ABS...weird. I think there is more vacuum to be had with a little more tuning. I talked to Booster Dewey in Orgeon, he rebuilds original boosters, but said the Chinesium ones cannot be rebuilt, their not quite the same specs as USA made. Ill probably send him my original to rebuild. You can Google him and see if he has a chrome original that would work for you.
#16
Im having the same problem with my aftermarket "original looking" power booster. More than likely its Chinesium, and not likely to be as good as an original. I did try an after market electric pump and reservoir which did not help. So i started jacking with timing and carb adjustments and got my vacuum up to almost 15" which helped some. It was at 12 inches before. It wont build much vacuum just idling but if i get it out and drive it, i have pretty good brakes, it wont quite lock em up but it will stop quick. Almost like ABS...weird. I think there is more vacuum to be had with a little more tuning. I talked to Booster Dewey in Orgeon, he rebuilds original boosters, but said the Chinesium ones cannot be rebuilt, their not quite the same specs as USA made. Ill probably send him my original to rebuild. You can Google him and see if he has a chrome original that would work for you.
#17
"You should have at least 18” vacuum to the booster. Anything lower will begin to give a hard pedal. If the vacuum level is below 18” you may be able to tune the engine and bring the vacuum level up to that level. If the vacuum level is around 16” the addition of a vacuum reserve canister will improve the braking. If the vacuum level is below 16” you will need to add an electric vacuum assist pump to supplement the engine vacuum."
What is your take on this?
What is your take on this?
As said, if your issue started with the new booster, then that is most likely your problem.
#18
The booster is simply a diaphragm inside a metal can connected to the pedal pushrod. With the pedal released, the engine evacuates both sides of the diaphragm. Since both sides are at the same manifold vacuum level, there is no net force on the pushrod.
When you press on the brakes, the CONTROL VALVE (note the diagram) opens and admits air at atmospheric pressure into the firewall side of the diaphragm. You now have 14 psi on one side and manifold vacuum on the other side. The result is a net force pushing on the master cylinder. If this control valve is damaged, sticky, or otherwise isn't admitting air as designed, the result is low or no power from the power booster. Bad control valves in brand new shiny chrome Chinesium brake boosters are not unheard of.
Also, since the mind reading still isn't working, it would help to know EVERYTHING you changed on the car since the last time the brakes worked properly.
#19
Gents - can defects in a booster cause the opposite effect? My '71 cutlass rear drums are fine at light to medium-light pressure, but once you get past that they grab like nuts and often lock up - no controlling it. Heaven forbid you have a hard stop. But perfect otherwise. Some superficial reading made me think gummy/sticky shoes, but the more I read about vacuum and braking, the more it sounds like the issue is more likely related to vacuum. I have good vacuum (18) at manifold, but PO removed vacuum canister on firewall if that matters. Can you recommend next steps for diagnosing?
#20
Yes, a failing booster can do that.
Back in the 90s I had a booster going bad and it would be very grabby like that for the first two or three stops then the pedal would be hard after that.
The vacuum canister on the firewall is for the HVAC system. It provides a vacuum reservoir for the various under dash duct controls. Without it, you get air blowing out of the floor vents only.
Back in the 90s I had a booster going bad and it would be very grabby like that for the first two or three stops then the pedal would be hard after that.
The vacuum canister on the firewall is for the HVAC system. It provides a vacuum reservoir for the various under dash duct controls. Without it, you get air blowing out of the floor vents only.
Thread
Thread Starter
Forum
Replies
Last Post
Dead Reckon
Small Blocks
18
July 14th, 2014 02:35 PM
zodiacblueS
Brakes/Hydraulic Systems
0
May 21st, 2012 03:45 PM