Hello:

I am new to this group as I just acquired my first Camaro a couple of weeks ago. It is a 1969 RS/SS 396 that has been restored to original condition.

Today a strange thing happened with the brakes (power assist with front discs and rear drums). I was driving along the road at about 50 mph. As I approached my destination, I pressed the brake pedal lightly to slow for my turn. All of a sudden the brakes locked up and the pedal stayed down as if some invisible foot was pressing on it. Luckily, nobody was following behind me. After about 2-3 seconds the brake pedal slowly raised back to its normal position. I tried to get it to repeat this, but the brakes functioned normally once again. Then, later this morning while I was driving somewhere else, the same thing occured. This time I was driving along and just applied the brakes lightly to slow down as I was coming into some traffic. The brakes locked and I steered over to the shoulder, again lucky nobody was close behind. As before, the brake pedal slowly raised back after a couple of seconds. I drove home this evening and didn't have any problems at all.

What can I do to begin to diagnose this problem? Master Cyclinder, Vacuum Booster??? Sooner or later, someone is going to slam me if I lock up the brakes in front of them! Any and all help will be greatly appreciated.

Phil

I'd suspect the brake booster. If it is an old one, there is a filter pad around the pushrod to the pedal, I have seen these pads rot and maybe chunks of it have gotten sucked into the control valve inside the booster, holding it open. When you step on the brakes, the "reaction valve" opens and allows atmospheric pressure air in, and it presses on the diaphragm.

It might be something else, but I'd look at that first.

Thank you for the reply. The booster, and master cyclinder for that matter, appear to be new, or recently replaced. Is is possible to see the filter pad from looking under the dash?? Is the "reaction valve" you mentioned something that can be examined visually?

Thanks again!

Thank you for the reply. The booster, and master cyclinder for that matter, appear to be new, or recently replaced. Is is possible to see the filter pad from looking under the dash?? Is the "reaction valve" you mentioned something that can be examined visually?

Thanks again!

I doubt it but you can try maybe using a small mirror around the brake pedal pushrod. The filter pad he is talking about is recessed in the back of the booster and there is not much room under the dash.

Personally I would just pull the booster and inspect it as you definitely do not want to drive the car anymore until you find the problem.

DITTO!
Personally I would just pull the booster and inspect it as you definitely do not want to drive the car anymore until you find the problem.
IMO, it got to be the booster. Also check the basics, as if the rod is attached to the lower hole on the pedal assembly. Good luck and be careful.

Thank you all for your help and information. Earlier today I spoke with a reputable Corvette restorer in my area (I have a 67 vette that he helped restore). He felt the problem was most likely in the proportioning valve. He explained that if there is dirt in the valve, it could dump all the pressure and fluid to the fronts locking them up. (I can't say for sure if all four brakes locked or only the fronts since it happened so unexpectantly.) This dirt in this valve could account for the random occurance of this problem.

What does anyone think of the possibility of the proportioning valve???

Phil

I'd say listen to David Pozzi and the rest of the guys here on this one.

FWIW I have leaned on David a number of times.... Hasn't steered me wrong yet!

Ditto.
I cant think of any way that dirt or blockage anywhere in the valves or lines could cause your pedal not to return. The master has compensating ports to compensate for any fluid not returned, even if the wheel brakes remained locked. the pedal return is all in the booster and master.

Ditto.
I cant think of any way that dirt or blockage anywhere in the valves or lines could cause your pedal not to return. The master has compensating ports to compensate for any fluid not returned, even if the wheel brakes remained locked. the pedal return is all in the booster and master.
Ditto. The fact that the pedal would draw itself to the floor and remain there momentarily could not be caused by dirt/blockage in the proportioning valve.

Once again, thank you all for your help. I too agree that my first inclination was that it had to be a problem with the booster. I'm planning on taking it off to inspect it over this weekend. It looks to be a relatively new unit, so I doubt that the filter pad could be falling apart. Other than the filter pad, what else can I look at while the booster is off?? Is there a way to "bench" test it?

What about the master cyclinder? Could there be trouble there?

Phil

I always bench test a new booster before installation. Use any driver vehicle and run a hose from a vacuum port on the intake to the booster. Make sure you have the short master cylinder pushrod in the front of the booster to prevent vacuum leaks. With the engine running you can activate the booster with hand force on the back pedal linkage. You can move in and out with ease and observe its operation. It should hold vacuum after the hose is disconnected from the check valve. You can get a couple of pumps after the vacuum has been disconnected. It will then be very hard to operate without any vacuum.

The filter pad is around the pushrod, you can probably see it frum underneath. Some guys that convert from manual to power brakes install the manual brake boot on the power system. DON"T do this! air needs to freely flow into the rear of the booster.

The valve inside the booster is plastic, it can be easliy broken if the pushrod is move to one side during shipping or handling it.

If the brake pedal cannot return fully to the rear, it can cause something like this according to reports I've read here. The booster should control pedal return height, not a rubber bumper or stoplight switch.

There are two depths of holes in the rear of the master cyl and two lengths of pushrods sticking out of the engine side of the booster. This usually causes a very low pedal and might cause sudden application of the brakes.

David:

Thank you for this valuable information. I'm wondering if the car was converted to power brakes, and consequently did the previous owner re-use the manual master cylinder? I does appear that there is a white plastic "boot" between the booster and the firewall....I gather from your post that this should not be there?

Is there any way to check to see if the valve inside the booster is okay, or do I have to send it out to a repair shop?

Could you please explain a bit more about the two lengths of pushrods to the mc? Should they be specific dimensions that I can measure just to check?

Thanks again,
Phil