[Will]

I know there is a dual-diaphragm booster that can be used in these cars. I believe it's also a smaller diameter, right? And it works with lower vacuum than the single-diaphragm boosters?

Set me straight on what the original application for these boosters was, what size they are, what kind of vacuum levels they'll work with compared to the single diaphragm units, and anything else I should know or any reasons to (or not to) put one in my '73 'bird.

Thanks!

[Scarebird]

1981 301 Turbo with rear disc.

https://www.rockauto.com/en/moreinfo...250873&jsn=437

Good to 9" Hg. You will need to mod the rod a bit.

[Geoff]

Dual diaphragm boosters [ DDB ] do not work on less vacuum. The benefit of the DDB is that the same vacuum operates over a greater area of diaphragm, & this is where the extra assist comes from.
A single 11" booster has theoretically 95 sq in of area for the vacuum to act against; a dual 9" booster has 127" sq in, a 33% increase.

[Scarebird]

er, no.

Your own math shows the area increase, also showing that 33% less vacuum is required to do the same job.

I have personally fitted these to cars that the 11" was not doing the work and the dual unit fixed the issue.

[64speed]

Scarebird what kind of mod needs to be done to the rod to get it to hook to a 64 Tempest pedal? Can you cut the end off and thread the clevis onto it? My brakes never felt like they were giving it all they had with my lowly 326 pulling 19 pounds so I’m sure they aren’t gonna be any better

[Will]

Thanks Scarebird.

Didn't the '81s also have an aluminum master cylinder with a plastic reservoir?

Any insight on the type of mod required? Rod to long, too short, wrong pin hole diameter, etc?

[Geoff]

Er, Yes. Scarebird.

The reason the DDB fixed your problem was because of the increase in diaphragm area [ 33% in my example ]; the same amount of vacuum was applied to both boosters, but over a greater area in the DDB. If somebody made a single diaphragm 13" booster, a similar increase in output force could be obtained.

[Scarebird]

yes, Geoff - I am baffled as to why you think the dual's do not work better as they have more surface area as you note. We are constantly plagued by braking issues from customers who use those $@#% 7" single "boosters" and wonder why the pedal is worse than a manual pedal - size doe matter here.

64- yes, that will need to be done. Not sure as the the angle fitment on a 64.

Spark - they can take either iron or aluminum. Talk to Booster Dewey in PDX, he really knows his onions concerning boosters

[Geoff]

Scare,
You need to read my #3 post again. Never said DDB did not provide more assist, I said the exact opposite in the example of the booster sizes I used as examples! But the booster size figures into the equation.
A 11" single diaphragm booster has roughly the same diaphragm area as a dual 8" booster, so they would provide about the same amount of assist for the same vacuum. Where the 8" booster might be a benefit is for valve cover clearance. A friend is having this issue with a 68 FB.

I think we are on the same page!

[Scarebird]

Case 1: 1969 Lemans running a 428 with <10" in Hg with a correctly operating stock 11" single booster - pedal was stiff and braking insufficient. We substituted a dual 9" from a 1987 S10 with adapter brackets using the same master cylinder - issue solved.

Case 2: 1955 Chevy with nasty 327 - same issue and solution, with same result.

Case 3: 1957 Buick Roadmaster with every option. Customer had rod shop install our conversion but used a single 7" booster - we get blamed for poor braking. I contact shop with issue, they replace with dual 9", problem solved.

We have more but won't bore you with them. The simple fact is that the dual 9's have more surface area and are a newer design (think better valving). A few years back we contacted Booster Dewey (a national-level expert) and the dual 9 was the best booster to recommend for ability/price/availability to our customers.

You have an opinion but it is just that - our customers deserve correct information and I do my best to provide that.

[MNBob]

I have the 81 master and booster(pictured above) and they work great. I did not have to do anything to the rod. My car was originally a 4 wheel disk car.

The 79 iron master was 9.6 pounds and the 81 aluminum 2.6 pounds. I eliminated all iron/steel in the braking system and now the fluid stays clear instead of turning dirty brown.

[Formula8]

It appears the dual booster is deeper than a single booster. My question is would not this push the master out further from the firewall? If so, do you need to use a later model rear brake line?

[MNBob]

The front and rear ports are reversed on the 81 master from the 79 so that is a consideration as well. On my 79, there was a big curly of lines with extra length so you might be able to make that work. I ran all new custom lines in NiCopp and stone guard.

[Geoff]

Scarebird.

I really don't know what you are going on about. The cases you you provided showed that when boosters were used with increased diaphragm area, braking was improved. That is exactly what I said in my original post.

[MNBob]

Originally, there was a metal bracket that mounted between the master and booster that held the fixed proportioning valve/distribution block. I thought that location was always in the way, so I moved it.

Here are before and after photos:

[url=https://flic.kr/p/26bTecr]
Original Master

[url=https://flic.kr/p/235o6MK]
81 Master and Booster

[nas t eh]

Originally Posted by Geoff

Dual diaphragm boosters [ DDB ] do not work on less vacuum.

This part of your first post is not the best way to state the point you are making. I have the same confusion about what you are trying to say as Scarbird.

What you might of said and Scarbird understands is that at every vacuum level the DDB will have more assisting force applied to the brakes. Since we only have a problem at the lower vacuum levels, that is all we care about. So they do work better at less vacuum.

[Will]

Originally Posted by MNBob

Originally, there was a metal bracket that mounted between the master and booster that held the fixed proportioning valve/distribution block. I thought that location was always in the way, so I moved it.

Here are before and after photos:

[url=https://flic.kr/p/26bTecr]
Original Master

[url=https://flic.kr/p/235o6MK]
81 Master and Booster

Nicely done! I'm not planning to replace brake lines but if I was, I'd probably copy that.

[Geoff]

nas t eh,
All boosters work on vacuum. To say that DDB boosters work on lower/less vacuum is incorrect....it depends.

If the combined area of both diaphragms in a DDB exceeds the area of the diaphragm in a single diaphragm booster [ I gave an example of a 33% area increase increase in an earlier post ], then yes, expect more assist from the DDB with less vacuum.

If the combined area of the diaphragms of a DDB is less than the area of a single diaphragm booster, then the assist from the DDB will be lower for the same vacuum, & the assist will be lower still if the DDB was working with 'less vacuum'. An example of sizes for this scenario would be a single 11" booster & a dual 7" booster. The 11"booster has 23% more diaphragm area over the dual 7"booster.

So the blanket statement that DDBs work on less vacuum is simply incorrect without knowing what booster sizes are being compared.

I don't know why this is so hard to understand, it is just high school physics!

[Rick Jones]

I have an old '81 booster and master cyl. I was considering putting on my '71. How do you test a booster to see if it is good?

thanks, Rick

[critter]

Will, I used the 79 4 wheel disc brake proportioning valve and dual diaphragm booster on my car when I swapped the rears for disc. Works well doesn't look as out of place under the hood of an earlier car. The biggest difference is the diameter of the booster. I did drill the brake pedal support and move the mount point about and inch higher. I'm not sure that was absolutely necessary. As for vacuum, I can't say it was any better with a big cam. My motivation was more for moving to 4 wheel disc.