This wont be for everyone so....tread easy ok? I had terrible brake pedal feel like I had almost no assist. I have an excellent pedal feel now and still have both engine vacuum to the brakes and the electric vacuum pump as a fail over design.

I have tall valve covers on a big block and so installed a 7" dual booster in my car back a while ago when everything was out of the car. My engine only produces 16" of Vacuum and thats not enough for even a 9" booster technically. (Brake manufactures recommend no less than 18" vac for booster assisted cars).

At the time I wanted power brakes and I wanted a big block (that was 11 yrs ago) and I was in the middle of a complete body off . Yes a manual master would work fine and yes you could find a set of valve covers that would clear the 9" booster all things I didn't do. If it was a few years later I would have gone LS engine instead...

For those interested this is what I found and I am really pleased with it. I didn't want to pull the drivers dash again and removed the booster in lieu of a manual master so this was more fun..... hope this helps someone else .

• ACDelco 178-0921 vacuum pump hose (Chevy cruze turbo based cars)
• https://leedbrakes.com/i-23439165-el...it-series.html ( I didnt like Leeds one wire setup and wont install any electric motors in this car without a relay and power to the Batt directly so I wired up the hella my way take it or leave it..)
• HELLA 007794301 Cover with Weatherproof Mini ISO Relay
• HELLA H84709001 Pig-Tail Wire Harness for Mini ISO Weatherproof Relays
• Blue Sea Systems Waterproof in-Line ATO/ATC Fuse Holder
• scrap metal to weld to the leeds pump mounting bracket.

Best place I could find was the #2 body mount at drivers feet. Pump weighs about 5 lbs so i wanted a stout mounting location. The pump is very quite rotary vane NOT piston based... you barely can hear it run and NO vibration at all. Vettes have NO under hood room so this worked great. I also dont have the factory vac tank. I only have the vac wiper door.










I wanted to have both the vacuum pump and the engine vacuum as a backup in case one failed. I found that Chevy cruze turbos use the same pump and have the backup failover / design i wanted as well as all the one way check valves built into the booster port adapter. I removed the factory Cruze hoses to get down to the twin port vacuum adapter and just tied my engine vac and electric vac pump hoses to it.

 

I hate to tell you this, but 16"Hg is plenty to run a standard power brake booster, if:
1. The booster is fed by a large fitting at the base of the carb with no other attached vacuum lines;
2. The booster check valve is operational;
and
3. The booster is in good condition.

I sincerely hope you are not going to the trouble of buying and installing an electric vacuum supply due to one of those listed reasons being faulty.

 

16" is not enough on a 7" booster... not even on a 9". Go to any brake manufacture site and ask them.

Do you really think I would go through this if 16" was enough? I did myself in with the small booster and really didnt want to pull the dash apart to get to the upper left nut again.

I couldn't lock up the brakes before ... now i can.

Like I said in the post.. your mileage may vary... in my car 16" on a 7" booster was simply not enough. Around town easy driving the pedal was ok any kind of spirited driving or panic type stops and it felt like I came up against a wall on the pedal especially in turns. My booster would hold 16" overnight and not drop at all so i had no leaks and I have no small fitting or restrictions. I also have a small vac reserve tank that I tried prior to the pump.

I called the manufacture of my booster and master and they told me 18" was the minimum... so thats what I had to go by.

 

Good job!!! Pretty brave saying anything about the brakesystem around here, some people will want to burn you as a witch.

I did something similar in my 68. I only had 6 to 8 inches at idle, I used a Hella rotary vane pump but had the big power booster so I only needed 15"hg. Nice and quiet too and it came with the 15" vacuum switch. I guess I could get a higher cutoff switch if it was necessary for my system but it works great as is.. I bought a handful of one way valves and put the pump on the wiper/headlight vacuum side with one way valves going to the brake side and one ways at the engine vacuum ports. If anything fails on the wiper side, the brakes are unaffected. if the engine stalls I have more than one chance to coast and stop the car and still have enough vacuum to stop without having to stand on the brakes with both feet. It works great and I have more than enough vacuum at start up without having to rev the engine to build vacuum....

 

You do really nice work! But I would tend to say that such high vacuum is not required. I don't think that I have had a motor in my vette for 35 years and 178,000 miles that has produced 12 inches of vacuum even at 1200 rpm idle speeds because of big solid roller cams. I've had the opening headlight not work from lack of vacuum, but never the brakes. Pad compound is a big factor. The super clean wheel ceramic pads that you get cheap at the local parts store might have very low brake TQ. I have tested all kinds of pad compounds out road racing and failed stock brakes even with 600 degree fluid and never with high vacuum engines

 

"some people will want to burn you as a witch" This made me laugh ... thank you for that!

I tried to put a disclaimer at the beginning... everyone's results and past experience will differ. Heck 70's chevelle had 11" diameter boosters (albeit single diaphragm).... I have all new hoses etc of course NONE of it is stock. The point is I wanted to improve on a sketchy 7" install.. I just should have known it wouldn't be enough live and learn. All i know is this works and has fail safe built in and I hate touching the drivers side dash...

 

Txs George... I certainly cant question your experience and admire your technical posts ... a lot of good stuff from you over the years!

The killer on this was my early move to a 7" booster for valve cover clearance... a couple years later and i would have gone with an LS. Ive only got 1200 miles on it since all the work... better pedal feel and brake performance went to the top of the list and I am not as nimble under the dash as i used to be... the freaking pedal feel in turns after lifting off the gas was terrible almost like loosing all assist... i hate touching the drivers dash or I woulda gone to a manual setup. LIve and learn. When I called the manufacture about my issue his first question was how much vacuum.

I dont know what type of pads I have now.. I dont think they are ceramic cause the wheels get pretty dusty... ill have to source an new set.

 

"Burn you as a witch...." Well, we could 'namby-pamby' around for a while so your feelings wouldn't be miffed, but I thought your post on an OPEN forum would allow you to hear something useful, rather than just what you wanted to hear.

I'd like to hear from those who have a 7" booster and have absolutely no problem with 15" Hg vacuum. I'm sure there are plenty of them......

 

I'm referring to using an electric vacuum pump.......there was a thread last year where everyone was saying the poster was going to kill himself if it failed....everyone knows you need more vacuum with a smaller diaphragm, that's just physics...lol
I did a search and what the manufacturer's recommended was actually above 18" hg in most threads that mention it.
I found one that was 23". Now what they recommended and what they will still function at effectively was another topic that kept popping up.
And remember at higher revs and decelerating your going to pull more vacuum than at idle and the booster can store that vacuum, but it will get used in one pump then it'll have to replenish with what your engine is idling at so subsequent pedal pressure will increase each time...as the vacuum decreases.
More is better....here is just a sample
https://www.chevytalk.org/fusionbb/s...hp?tid/280336/

 

I learned something from chevy talk...… I never knew that 7 inch booster were so marginal when it comes to repeated braking.

A double 8" booster has a boost rating of 101 while a single 7 has only 38. This is way too small. A single 9 has 95. Wilwood suggests a master cylinder with a bore of 7/8" to 1" dia. You could contact them for more info or Ralph at ECI (860-872-7046, www.eciHotRodBrakes.com) who knows brakes inside/out. He probably has the exact answer for your 4 wheel disc needs.

 

Hey guys I appreciate EVERYONES experience and input.. your absolutely right this is an OPEN forum and im sorry if I insulted you on your comment. I should be more angry at myself and my choices and not take that out on anyone else... Hope to learn this someday before I leave the planet. We all bring years and years of experience and any comment could percolate up something we haven't thought of.

But, i have to chuckle at the side comments we get too... There are sooooo many times I hate this car and then I take a ride in it and it all goes away. Or i see a phrase someone uses and the visualization that comes to mind just cracks me up...

Keep em coming guys I really appreciate you guys taken the time to look, research and comment. Its my therapy on this frustrating trip.

 

Hmmmm mines a double diagphram 7" i wonder if they have specs on boost rating for that. Again, you guys come through with research I should have done before swapping to the 7".

I should rename this thread to "7" booster + < 18" vacuum = You MAY need a vac pump?

This all started when i drove my buddies 68 with assisted brakes and I like his pedal feel. Mine felt like a drum brake nova compared to his....