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Discussion Starter · #1 ·
Hello -

I have a 1969 RS with what appears to be factory power front disc and rear drum brakes. Over the last several weeks I noticed that my car does not seem to roll as easily. I jacked it up today and the wheels rotated fine with the engine off. I next started the engine and then I could not rotate the front wheels by hand at all. The brakes were being applied. I also noticed that the brake pedal depresses by itself about 0.5" when I start the engine. I noticed this as I was installing a new brake light switch. I also heard a hissing sound coming from the booster area inside the car. The vacuum hose was connected fine at both ends.

Any idea what is wrong? I replaced rotors, calipers, wheel bearings, etc. and the brakes mechanically are fine. I just don't understand why when the engine runs the pedal depresses by itself and applies the brakes.

Please help.

Thank you.

Hynek
 

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Look where your brake booster pushrod enters the firewall. There is a foam pad filter there that keeps junk from being sucked into your booster. The foam can rot and then chunks of it can be sucked in and get caught in the internal Reaction Valve.

When you step on the brakes, a vacuum "Reaction Valve" opens and let's atmospheric air enter which pushes on the diaphragm. YOu either have to dissassemble the booster and clean it, have it rebuilt, or buy another one.
 

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Isn't there a small spring that mounts to the top of the pedal assy for the brake? If it's not there, maybe the weight of the pedal is enough with the car running to engage the system.
 

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Discussion Starter · #4 ·
Thank you for the responses.

I didn't think there was a spring holding the pedal back originally so I am not certain. Also my brake pedal has only one hole for the clevis pin. I read many posts about using either the top or bottom hole.

How do I know if my hole is in the one for power brakes and not manual brakes?

I am not 100% certain my car had factory power brakes.

Thank you again.

Hynek
 

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I'm pretty sure only manual brakes used the return spring.

Power brakes use the bottom hole. If your pedal only has one hole, that would leave me to believe someone changed the pedal. If you'll notice under the hood, your booster puts the master cylinder at angle relative to the firewall. By using the lower hole on the pedal, it makes the rod enter the booster more or less straight, while at the same time giving less mechanical advantage than the manual setup. Hopelly someone with a power setup can measure how far down the hole is on the pedal. I'd do it for you, but I'm at work today.

Another thought, regarding your problem, is that the rod between the booster and the master cylinder is slightly too long. You can remove the two nuts holding the MC to the booster and place a couple of thin washers between the booster and the MC to space it outward a little - don't forget to reinstall and tighten the nuts - that could help diagnose that possibility.
 

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Discussion Starter · #7 ·
Actually my pedal always felt very hard and was very touchy. I always contributed the performance to low vacuum.

I'm still confused about only one hole in the pedal - does this mean it was a factory manual brake car?

Also does any one have measurements on the hole spacing so I can tell if I have the "higher" hole or "lower hole"?

Thank you again.

Hynek
 

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Look above the clevis for the booster, there should be a metal tab that contacts the brake light switch, it is held by a bolt in the "upper" hole.
The power brake hole is 3.25" down from the pedal pivot.
There is no return spring on power brake cars, the booster spring is very large and takes care of pedal return.
 

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Discussion Starter · #9 ·
Thank you once again for all the information. I do not have a tab in the top hole which leads me to believe I have a combination of issues. (David - I frequent your site often and saw the photo of the metal tab) I will take the measurements and correct the clevis position.

I will update this thread on what I discover so as to help others with this issue.

Thank you again for the information as this is such a great technical community!

Hynek
 

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This is for anyone working on a brake booster:
The reaction valve inside the booster is plastic and can break if the pushrod is bumped or pushed to one side too far. When you have a booster out of the car, take special care of the pushrod.
David
 

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Discussion Starter · #11 ·
Hello -

I need to correct myself as I have supplied some miss-information. Here are the facts.

1) Brake pedal DOES have two holes and the clevis pin is in the lower hole.

2) I removed the master cylinder from the booster today and noticed I have a "deep" hole master cylinder. I removed the push rod between the master cylinder and booster and it was the short push rod. I saw from reading the brake info on David's site that this is not correct. I have driven the car like this and had brakes previously - how is that possible? Could the short push rod in a deep MC be causing the pedal to lock the brakes when the engine is running?

I received the new booster today from Rick's and am ready to replace. I would rather not though if it was only due to a short push rod in a deep MC.

I'm just wondering what this mismatch would have caused - is the root cause of my locking brake problem?

Thank you in advance.

Hynek
 

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I had a smilar problem. I just installed a MBM power/disk conversion on my 67 Firebird. My brakes preloaded or just got tighter and did not release as I test drove the car around the block.

My kit came with two different sized adaptor pins. The pins went between the booster and the master. I installed the smaller of the two pins since it seemed to fit better. The problem was I was NOT supposed to use ether adaptor. According to the MBM website, the booster part of the kit comes with the adaptors so you can fit the booster on a "deep" master. The deep masters tend to be on manual systems. Since my kit also had a new "shallow" master, I did not need the adaptor. I pulled the adaptor last night and the car works perfect.

Maybe this will help you?

Also, try googling MBM Brakes and look at the tech sheet.

Good luck
-Rob
 
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