Our all drum 67 has very strong brake boost. We would like to lessen the assist as it is very very sensative,  one toe braking.
How much difference regarding the boost would change on switching to a smaller booster?
Our booster is getting a good 20 inches vac from our bone stock 326.

Can I ask: is that the factory drum brake booster? Disc brakes require more pedal effort in order for them to work and so have a more powerful booster than a drum system. A friend used to have a drum braked Falcon and he was going to change to discs, so the first thing he did was to install a disc specific booster. He drove the car around until the discs were fitted but the front brakes would lock up by simply looking at the brake pedal! I never forgot that because I couldn't believe the difference in the systems. My thinking is that you shouldn't have to go to a different booster provided all your brake bits are as they should be specified from the factory.

Thank you Fitzy. You bring up a point that is very interesting. I actually don't know anything about this booster as it came on the car from a seller out of state.

I now  have a new question after researching this for a while.
I see listed for a 1967 Pontiac Firebird (4) different brake boosters with seemingly no differentiation I can find (which really makes me wonder if my booster is correct for my application).

One is 11" single diaphragm, one 11" dual diaphragm, one is 9" single diaphragm, and one 9" dual diaphragm - all being listed for 67 firebird .

Edited April 16, 20233 yr by Hotrodjohn71
Spelling

Sounds like you need to research exactly what booster your car is supposed to have. Ames is a good place for this - you can email their tech dept. They have always been helpful for me.

10 hours ago, Hotrodjohn71 said:

Thank you Fitzy. You bring up a point that is very interesting. I actually don't know anything about this booster as it came on the car from a seller out of state.

I now  have a new question after researching this for a while.
I see listed for a 1967 Pontiac Firebird (4) different brake boosters with seemingly no differentiation I can find (which really makes me wonder if my booster is correct for my application).

One is 11" single diaphragm, one 11" dual diaphragm, one is 9" single diaphragm, and one 9" dual diaphragm - all being listed for 67 firebird .

It looks like your booster is an 11”. That’s to big for 4 wheel drum. You need no more than a 9” single diaphragm booster for that application.

John, i have a question for you ...

when you brake hard are the front brakes locking up? even to the point the steering wheel gets riped out of your hands while turning  ?

Last indian is correct, your car should be 9inch booster, and it certainly looks to be a 10.630

Reason i say that is that master cylinder does not look correct for your car !

 

 

Edited April 17, 20233 yr by 64 kiwi boni

Disc Brake master cylinder have a bigger bore for the front when using discs

Hi Kiwi, if I do a panic stop, the fronts will lock up, but I think all the wheels lock up as well.

Our m/c is a 1" bore which is correct for power (all drum).

Manual (all drum)  are 7/8" on the 1967 Firebird.

For the power drum/disc cars, it's a 1 1/8" bore m/c.

Edited April 17, 20233 yr by Hotrodjohn71
Size, punctuation

and have you done any work to the drums and shoes ? 

everything else sounds correct .. unless the proposioning valve is sticking 

Hi Kiwi, all the brake drum assemblies are clean, dry new and adjusted correctly.  All wheels are adjusted so they will spin about half way around.

On this 67 Firebird with power drums, they did not actually have a proportioning valve. They do have a distribution block which is just a 4 way junction block with a switch to the BRAKE warning light which only serves to show a differential between the front brake circuit and rear.

4 hours ago, Hotrodjohn71 said:

Hi Kiwi, all the brake drum assemblies are clean, dry new and adjusted correctly.  All wheels are adjusted so they will spin about half way around.

On this 67 Firebird with power drums, they did not actually have a proportioning valve. They do have a distribution block which is just a 4 way junction block with a switch to the BRAKE warning light which only serves to show a differential between the front brake circuit and rear.

thats another box ticked John, we are running out of ideas..... i have had a Plymouth with what i call sticky front brake drums even after replacing the shoes it still was sticky to the point a wheel would lock up, and i had to replace the drum itself.

question for you, have the front drums been reserfaced ? and if so how close to maximuim are they ? if they are close to max diametre thay can move around and cause stick braking.

Other than that i think for the cost of a new master cylinder, i would try a new drum drum unit. 

And justA for interest, try disconnection of your vacuum line to the booster and see how they behave !

Edited April 18, 20233 yr by 64 kiwi boni

On 4/17/2023 at 10:23 AM, Hotrodjohn71 said:

Hi Kiwi, if I do a panic stop, the fronts will lock up, but I think all the wheels lock up as well.

Our m/c is a 1" bore which is correct for power (all drum).

Manual (all drum)  are 7/8" on the 1967 Firebird.

For the power drum/disc cars, it's a 1 1/8" bore m/c.

Have you tried removing the vacuum line from the booster to make it a manual system to see how it brakes? A 1” bore is a big bore. And the 1 1/8” is huge, that’s in the realm of 4 wheel disc brakes duel piston! 

One last thought. I know you say your is standard at 1” for 4 wheel drum power, but that’s not what I remember. I believe they were a 7/8 piston period. The upgrade was a booster on a 7/8 master cylinder. 1” piston was for disc front drum rear & 1 1/8 for 4 wheel disc, which was only available for the Camaro Z/28 j8 option.

I will have to do a bit more research,  but according to this manual, for 67, the cars without DB had 1" bore master cylinder. 

 

I did an experiment the other day and unhooked and plugged the vacuum line to the booster and it was a great pedal, high and firm. But just lacking assist. Hard to push.

yeah i think your master cylinder size is correct.. i justA went and looked at the bore size on my master cylinder for the 64 which is 4 wheel drum and boosted ..

it is 1 inch ....

 interesting how you said it braked good with  no assit .... kind of points towards too much vacuum !  which is going back to What last indian said about booster size, gees its justA diaphram... theres nothing to them ! 

Thanks for the advice. I have an 8" delco style booster on the way. I will also recheck if the drums were new or turned. I am pretty sure they are new. But I will find out.

6 hours ago, Hotrodjohn71 said:

Thanks for the advice. I have an 8" delco style booster on the way. I will also recheck if the drums were new or turned. I am pretty sure they are new. But I will find out.

I believed you on the 1”, It’s just been a long long time & I had it stuck that I remember 7/8”. That said the 9” is the right size, but the 8” should be good too. On your brake pedal there should be two holes for the celvis pin. Which hole do you currently have it in? Also, how much free play do you have?

Thank you for all of your help. I have solved this breaking issue by installing a 9-in Delco Moraine brake booster. I have a good and high pedal now that is not grabby or extremely sensitive.

That’s great news John 

Now post us some pictures of you gettting out and enjoying that cool car 👍👍👍