Don Roberto

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.

joe_padavano

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.

Don Roberto

Yes, it is a chromed Delco aftermarket. No, no issues before with stock. I can try with a new check valve, but the booster is holding 15" as of now. So it could be still a check valve not working?

oldcutlass

There has been a rash of aftermarket booster failures lately. The minimum rule of thumb on most booster vacuum requirements is around 13"'s. You don't have a vacuum issue.

Don Roberto

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.

Don Roberto

[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.

oldcutlass

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?

Don Roberto

I have not checked that! It could be that simple. I will check it out tomorrow and report back.

Don Roberto

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?

joe_padavano

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...

Don Roberto

Ok one might get to the conclusion that the aftermarket booster is defective. However, the booster holds 15" of vacuum with the mild cam and performers intake. The hardness on the pedal is slight, but no, the brakes are not locking as expected for an emergency situation. I got the following info from the Jegs general brake booster installation instructions:

"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?

classicmuscle442

Think the guys here know there Oldsmobiles, you need a factory style booster, problem solved.

oldcutlass

It also has a tendency to bind the rod in the upper hole and damage the bushing in the booster.

Don Roberto

This is what I have. 11" AC Delco-Muraine Chrome power brake system.

steverw

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.

Don Roberto

Makes sense! Do you a performance cam as well?

Fun71

My take - BS. My engine makes a steady 16" vacuum and the brakes work fine. Previously, the vacuum was 14-16 and bouncy and the brakes were marginal, so I did some distributor tuning and got the vacuum to its present level.

As said, if your issue started with the new booster, then that is most likely your problem.

joe_padavano

The fact that the booster "holds" vacuum only means that it doesn't leak. I suggest you read Chapter 5 in your chassis service manual that describes how the booster works.



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.

sheltonoil

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?

Fun71

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.

sheltonoil

Thanks. So should I get to an empty parking lot, disconnect vacuum line from booster, make a few hard stops and see where that gets me?

oldcutlass

If your pedal is not hard, most likely the booster is good. Chances are you have/had a leaking wheel cylinder and saturated brake pads.

sheltonoil

oldcutlass

I meant pad material on the shoes...