[Hotrodjohn71]

What were the size options for a power brake booster on a 67 Firebird?
Was there more than one option?
The booster on our car has an incredible amount of boost.

[TedRamAirII]

No, Just one size, 11" .

[north]

That doesn't look like a factory booster. BTW, while all the factory boosters were one size that year the disc brake booster was different (different part number) than the booster for a drum brake car.

[Hotrodjohn71]

Thank you. I did neglect to mention that our car is all drum.
Thank you North. That is ab interesting bit of information. I wonder what the difference would be.

[Hotrodjohn71]

How much do you think I can lessen the boost by going to a smaller booster? Will there be a noticeable difference? I think mine is an 11". What about a 9" or less?

[north]

I am not an expert on this subject but I believe that boost level is related not to the size of the booster as much as internal orifice-restrictor sizing.

[Hotrodjohn71]

Ok. Thank you. I 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 .

[vertigto]

If you're looking to lower boost, I have this essentially new 9" Right Stuff booster that would not work with my setup. Seems to fit a 67 Bird.

https://www.jegs.com/i/Right-Stuff/9...10572/10002/-1

[JLMounce]

You want to try a single 9" (or 8" in it's available) in this application. The 11" and anything dual diaphragm is going to provide way too much boost in a full drum setup.

Was a power drum option even available on the first gens? I understand GM did that in the early 60's on big luxury barges, but I'd not heard of it being used on a much lighter F-body platform.

Another thing to look at here is which hole in the pedal you are using. It's hard to tell from the picture, but it looks as though the booster and master cylinder are level, instead of angled upward. If that's the case, it's possible that the upper hole with the higher pedal ratio is being used. Not only do you have a significant amount of assist from the large booster, but you've also got a lot of pedal ratio. All of that is working against you with drums all around.

[Hotrodjohn71]

Thank you JL.
The brake rod clevis is in the bottom hole on the brake pedal for power brakes.
The m/c is definitely tilted up. For that reason, I searched and found a NOS master cylinder with the bleeder valves on the master itself to aid in bleeding that air that can get trapped in the m/c.
As for original factory power drums on this car, I cannot fully say as I got the car as a project from out of state, 'seems' original.

[JLMounce]

Hotrod, that’s a better situation then. If you go to a single 9” diaphragm booster, you will reduce assist by roughly 33%.

That’s where I would start if I was in this situation.

What size bore is in the master cylinder? It’s probably a 1” or possibly a 15/16”. If you find after the booster change that you still have too much boost, you can go to a larger bore master cylinder. If it’s a 15/16” move to a 1”. If it’s a 1”, move to a 1.125” master. In each case you’ll increase pedal effort again by about 20-25%.

As long as the master is bench bled, tilting the master is fine as long as the fluid level is adequate.

[Hotrodjohn71]

Thank you to all for your helpful advice. The solution was switching to a 9-in single diaphragm booster.
Now I have a good pedal, not an on-off breaking from overboost.

[JLMounce]

glad to hear you're back on the road with better functioning brakes.