[poncho62]

Just a note.....the late 60s -early 70s Quadrajets had the booster vacuum in the intake behind the carb...in the manifold. That is likely why there is no port back there. Does sound like the booster is the problem....but why does the pedal go right to the floor?

In the original post it sounds like he said that upon starting the engine the pedal went right to the floor and stayed there.....My question is, did he push the pedal and it stayed there....or did it go to the floor on it's own?...That I have never seen.

[joe_padavano]

Originally Posted by poncho62

In the original post it sounds like he said that upon starting the engine the pedal went right to the floor and stayed there.....My question is, did he push the pedal and it stayed there....or did it go to the floor on it's own?...That I have never seen.

Again, if the booster air valve is stuck open, the booster diaphragm will be sucked all the way to the M/C side of the booster as soon as you start the engine (and thus see manifold vacuum in the booster). This will pull the pedal to the floor, whether the pedal was touched or not.

Originally Posted by powerrodsmike

...Show me one GM car stock equipped with a blower and a quadrajet...

Well, uh, actually, both the original turbo Buick V6 and the late, lamented turbo Pontiac 301 came with Qjets - and technically, a turbo is a "blower".
Of course, these were both suck-through designs so the carb was never pressurized. In both cases the signal for the brake booster was taken upstream from the turbo.

[powerrodsmike]

Originally Posted by joe_padavano

Again, if the booster air valve is stuck open, the booster diaphragm will be sucked all the way to the M/C side of the booster as soon as you start the engine (and thus see manifold vacuum in the booster). This will pull the pedal to the floor, whether the pedal was touched or not.

I see another issue, the booster with the bad air valve does not have the ability to pull a pedal to the floor, even with a full 20" of vacuum, not with a properly adjusted and bled master cylinder hooked up to a properly functioning set of brakes. That is why I asked the OP if the brakes worked at all in my first response..

Originally Posted by Joe Padavano

Well, uh, actually, both the original turbo Buick V6 and the late, lamented turbo Pontiac 301 came with Qjets - and technically, a turbo is a "blower".
Of course, these were both suck-through designs so the carb was never pressurized. In both cases the signal for the brake booster was taken upstream from the turbo.

I stand corrected, and I did know about the turbo buick, but didn't want to confuse the issue any further.


Typically a guy who says his motor is "blown" means one of 2 things....either he has a belt driven supercharger on top of his motor, or a severe mechanical mishap resulting in an inoperative motor.

So in the case of the turbocharger equipped cars, the vacuum signal was still ported to a place under the throttle butterflies, but not in the intake, where the vacuum source was lessened during periods of boost.


The OP needs to get a new booster, check the rest of his brake system, and he can find a vacuum source where ever he sees fit.


On one guys car with a B&M blower I drilled and tapped a hole in the top of the blower case just under the carb base for a vacuum port. It worked fine.



Later, mikey

[joe_padavano]

Originally Posted by powerrodsmike

I see another issue, the booster with the bad air valve does not have the ability to pull a pedal to the floor, even with a full 20" of vacuum, not with a properly adjusted and bled master cylinder hooked up to a properly functioning set of brakes. That is why I asked the OP if the brakes worked at all in my first response..

Yeah, I came to the same conclusion shortly after I sent that prior post. Good call.

[Marcus]

I think you nailed the problem I'm having with the pedal being pinned to the floor...as soon as I start the engine and without me depressing it, it sinks straight to the floor and won't budge. Also, I think that in original configuration the brake booster port was located in the stock manifold. There are 3 small ports at the back of my particular Q-jet but none look like they were used for the booster. Incidentally, i'm using a B&M belt driven blower. Thanks all for your suggestions of this bewildering problem I had with my brakes.

[grandpaws 57]

No place for bad feeling. Stock quadjets[sic] hook pcv to front , brake booster to man . vacume. Get a check valve from boneyard,most likely problem

[powerrodsmike]

Originally Posted by grandpaws 57

No place for bad feeling. Stock quadjets[sic] hook pcv to front , brake booster to man . vacume. Get a check valve from boneyard,most likely problem

Please note that the original poster of this thread has a belt driven B&M blower installed on his motor. The fact that his motor is "blown" was stated in his original post.

You don't hook any vacuum operated component to the intake manifold on a motor equipped with a supercharger.

Supercharged motors have pressure in the intake manifold instead of vacuum during periods of boost, this would pressurize the vacuum side of the brake booster, rendering it not only useless, but making the brake pedal extremely hard to push. (this would happen the first time you pressed the brake pedal if the check valve was bad, most likely the second time if it was good.)

Correcting misinformation should not create bad feelings, it is intended to educate those who are able to learn.

Later, mikey

[joe_padavano]

Originally Posted by grandpaws 57

... Get a check valve from boneyard,most likely problem

Feel free to try, but there is no physical way the check valve can cause the problem being discussed. Since it is on the vacuum side of the booster, the check valve is normally open in the direction of the manifold. If it were stuck closed, vacuum wouldn't be applied to the booster in the first place and the pedal wouldn't move. If it were stuck open, the function would be the same as if it weren't stuck.

Keep in mind that the primary purpose of the check valve is to maintain vacuum in the booster so that you have power assist for one or two stops after the motor stalls. It also has a secondary function of protecting the booster diaphragm in the event of a backfire.