# 9" or 11" Brake Booster?



## System (7 mo ago)

My 70 has manual 4 wheel drum brakes and I want to convert to a vacuum power master cylinder. I see both 11" and 9" versions available and warnings about checking for room if ordering the 11". Can anyone advice me on which way to go based on this pic? (Don't judge the crud... it was a barn find and I'm just getting started...  )


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## Baaad65 (Aug 29, 2019)

I used a dual 8" for clearance on my '65.


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## armyadarkness (Dec 7, 2020)

That's the cleanest "barn find" I ever saw.

I use a single 9", on my 67, with front disks and rear drums. The car stops like a dream... AND!!! You wont have valve cover clearance issues. If you intend to ever run roller rockers and poly locks, avoid the big booster!!!

On my old 67, I had to keep a stamped rocker on the last stud!


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## System (7 mo ago)

Excellent thoughts there! I'll stay with the smaller unit then. Glad I asked...


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## armyadarkness (Dec 7, 2020)

System said:


> Excellent thoughts there! I'll stay with the smaller unit then. Glad I asked...


Yeah, do a little reading and you find many folks who got screwed with clearance on the driver side valvetrain. Imagine having to remove your Master cylinder to adjust a valve.


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## System (7 mo ago)

I'm guessing then that if the car had factory power brakes, the booster was 8 or 9"?


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## armyadarkness (Dec 7, 2020)

System said:


> I'm guessing then that if the car had factory power brakes, the booster was 8 or 9"?


Factory cars didnt have roller rockers or poly locks, but it's such a cheap, easy, rewarding mod, that most people want them.

A stock GTO with low profile VC'c, was a pain to take them on and off. With tall stuff, it could be impossible.


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## armyadarkness (Dec 7, 2020)

This isnt the best angle to see, but I cant even pass a spark plug wire between my VC and booster.


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## System (7 mo ago)

Wow, that is tight for sure. My wiper motor is where your Mallory ignition is so fingers crossed that won't interfere... Hopefully the booster stands out far enough that it won't...


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## armyadarkness (Dec 7, 2020)

System said:


> Wow, that is tight for sure. My wiper motor is where your Mallory ignition is so fingers crossed that won't interfere... Hopefully the booster stands out far enough that it won't...


The booster to VC distance is likely the same on our cars. Yours would accept any booster... just not if you ever used tall vc's


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## Baaad65 (Aug 29, 2019)

Here's my '65 I know yours is a little different but I can get the vc off ok but with a 9 or bigger booster it would be really tough and mine are taller than stock for the roller rockers. Some advice after doing mine, don't go cheap and if the master is on an angle like mine have a good sealing cover, I've gone through three gaskets because of the flat cover leaking and stripping the paint. I had so many things to replace I didn't want to pay for Wilwood so I went with MBM Chinese junk from Pirate Jack and they gave up on me and had no solutions for the leaking, they blamed me for using dot 5 fluid which I didn't. So I called MBM and they said those flat cover masters aren't made to be on an angle so he suggested a cork gasket in addition to the rubber, so I glued the cork to the cover and with the rubber it's been good so far. Idk how Wilwood would work because it's the same design as what I have. I originally got straight brackets for the master but even the 8" booster wouldn't clear.


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## RMTZ67 (Mar 12, 2011)

Baaad65 said:


> Here's my '65 I know yours is a little different but I can get the vc off ok but with a 9 or bigger booster it would be really tough and mine are taller than stock for the roller rockers. Some advice after doing mine, don't go cheap and if the master is on an angle like mine have a good sealing cover, I've gone through three gaskets because of the flat cover leaking and stripping the paint. I had so many things to replace I didn't want to pay for Wilwood so I went with MBM Chinese junk from Pirate Jack and they gave up on me and had no solutions for the leaking, they blamed me for using dot 5 fluid which I didn't. So I called MBM and they said those flat cover masters aren't made to be on an angle so he suggested a cork gasket in addition to the rubber, so I glued the cork to the cover and with the rubber it's been good so far. Idk how Wilwood would work because it's the same design as what I have. I originally got straight brackets for the master but even the 8" booster wouldn't clear.
> View attachment 155005


Had to get the valve cover breather in the shot, didn't you.


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## Baaad65 (Aug 29, 2019)

Oh ya, it's been a hot second 🤣


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## System (7 mo ago)

Being the FNG, I'm missing the breather joke...


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## Sick467 (Oct 29, 2019)

System said:


> Being the FNG, I'm missing the breather joke...


 Army and Baaad go back and forth about posting pics...My tally had Army up by one with the third photo this week of his engine bay, BUT Bad is back in the lead...by one!


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## Baaad65 (Aug 29, 2019)

Army is my breather groupie 🤣


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## Wirepickler (Jul 3, 2020)

Original 70 brake booster


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## System (7 mo ago)

Looks like 11"? I'm looking all over online for a loaded unit and they are sold out in both sizes.


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## PontiacJim (Dec 29, 2012)

System said:


> Looks like 11"? I'm looking all over online for a loaded unit and they are sold out in both sizes.



Here is some Tech reading/info on selection/needs: 






Best Brake Boosters for Better Braking


Power brake boosters offer lower pedal effort and better braking characteristics, but which one is right for your hot rod, muscle car, or truck? We take the guesswork out of that question with our latest buyer's guide.




www.speedwaymotors.com










Speedway Tech Talk - Booster Adjustment


Tim talks about brake booster adjustment.




www.speedwaymotors.com


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## GTOTIGR (May 3, 2020)

Here are a couple of sources for information, restoration services and potentially complete units. I’ve used Steve in the past with great results. He used to advertise in The Legend but I no longer see his ad there.


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## Wirepickler (Jul 3, 2020)

Mike bought the business from Steve, and did the booster in my previous photo. Highly recommend and they sell them already done also.


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## Mine'sa66 (Oct 30, 2019)

I have an 11" on my '66. It's tight, but clears. Really tall VCs might not clear. I could get the VC off with the stock valvetrain, but if I had tall stuff in there, probably not.


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## System (7 mo ago)

I'll call Mike (Steve's) next week and see if he has what I need in stock. Thanks for the info and leads guys. Most helpful....


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## lust4speed (Jul 5, 2019)

The larger the cam, the less vacuum and the more powerful the brake booster must be. If you are running 17" of vacuum any size booster will work. Get down around 11" of vacuum at idle and things become critical. A couple of guys in our GTO club spent a bunch of time testing different boosters and here's the pecking order of boosters from the ones they tested. The two best were the 11" single diaphragm with the 9" dual diaphragm almost as good. With large cams the 8" dual diaphragm would sometimes produce a hard pedal, and the 7" dual just never did work that great. They didn't test any smaller single diaphragm units but I'd want good vacuum for those. Three of our guys gave up on the booster when they went to very large roller cams and have went with manual brakes. Again, with enough engine vacuum any booster will work.

There was a point in time about three years ago where three out of four aftermarket boosters were bad out of the box, and I don't think quality control has improved. If a new system seems like it might not be working as it should there is a good possibility that the brake booster is defective. Units had vacuum leaks and just did not provide much boost.


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## System (7 mo ago)

I had that issue with my 69 Camaro years ago. A simple vacuum storage canister solved it. I'm guessing that's still a thing if need be but it's not pretty or stock looking in the engine compartment....


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## lust4speed (Jul 5, 2019)

Forgot to mention that the 11" works with all factory valve covers. It will just touch most 2-1/2" covers, and you have to get out the hammer and dimple the 3" tall covers where the edge of the booster contacts the edge.


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## armyadarkness (Dec 7, 2020)

lust4speed said:


> Forgot to mention that the 11" works with all factory valve covers. It will just touch most 2-1/2" covers, and you have to get out the hammer and dimple the 3" tall covers where the edge of the booster contacts the edge.


But will it work with taller valve trains? IME, it's a combination of the taller VC and the Poly's. 

On my 67, I have to get creative, but on my older 67, with the 11" it was just no dice.


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## armyadarkness (Dec 7, 2020)

System said:


> I had that issue with my 69 Camaro years ago. A simple vacuum storage canister solved it. I'm guessing that's still a thing if need be but it's not pretty or stock looking in the engine compartment....


I run a vac can and 48" of hose! But it works fine. Car stops great with rear drums and a choppy cam


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## Khmike (Jun 21, 2021)

Following (mostly for the breather talk, why lie?)


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## System (7 mo ago)

I did talk to Mike yesterday. He said he was honestly surprised that there would be a clearance issue with tall covers and an 11" booster... but then he had to admit that he really wasn't a Pontiac guy so perhaps something is different between GTO clearance and BBC in a Chevelle. Anyways, he said that he could build me either a 11" or a 9" with a new Delco master that was sleeved in stainless. Same price either way... $875 plus shipping. I was a little taken aback but I guess it is what it is. He said he's going on vacation for a week so I have time to decide.


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## armyadarkness (Dec 7, 2020)

System said:


> I did talk to Mike yesterday. He said he was honestly surprised that there would be a clearance issue with tall covers and an 11" booster... but then he had to admit that he really wasn't a Pontiac guy so perhaps something is different between GTO clearance and BBC in a Chevelle. Anyways, he said that he could build me either a 11" or a 9" with a new Delco master that was sleeved in stainless. Same price either way... $875 plus shipping. I was a little taken aback but I guess it is what it is. He said he's going on vacation for a week so I have time to decide.


Yes there is a huge difference between Pontiac and Chevy engines. The clearances wouldn't remotely be the same


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## System (7 mo ago)

Oh, absolutely....


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