Brake booster size help

[Tiger Joe]

ok guys got a 67 c10. I'm trying to swap from an 11" booster to a smaller dual diaphragm and need some help . with a BBC in there the 11" booster barely fit so I wanted some more room.

Front brakes- converted to 5 lug with stock gm disc brakes
rear- stock drums. all totally new.

prior to now the truck has the stock booster setup from the a 72 disc brake truck.

last fall I bought a capt fab bracket to mount the booster closer to firewall. I also decided to upgrade to a dual diaphragm booster

I put a Wilwood master cylinder and adjustable proportioning valve kit on an 7" dual diaphragm booster.

Is a 7" dual diaphragm booster enough, or should I use an 8" dual diaphragm?

I've found some things online that said to calculate the surface area of the booster, and if you do that:
1-assume the factory is a true 11" booster (my measurements put it closer to 10")
11" booster- surface area of 95
8" dual booster- 100

2- if you factor in the factory booster does not actually have 11" of actual area
10" booster- 78.5
7" dual booster- 77

however, for the brand of booster I purchased (Tuff Stuff Performance) jegs website list 7" as disc/drum and 8" as disc/disc.

any help here would be great. If I can use the 7" dual I would prefer as it gives me the most amount of room

[adricar1]

booster size will be determined on the amount of vacuum the engine produces,if you post this then it will be easier to advise.

[Tiger Joe]

Originally Posted by adricar1

booster size will be determined on the amount of vacuum the engine produces,if you post this then it will be easier to advise.

I can try to get a vacuum reading tonight.

what should I be looking for? say idle and 1500-2000 rpm readings?

[Tiger Joe]

I was having trouble getting a vacuum reading last night.

BBC with no choke does not like to idle cold. my vacuum reading was around 12-15. rev it up a bit to say 1500 and I could see it jump up.

Right now I have the 7" dual diaphragm installed and it seems to work fine. I didn't want to pull out of the garage since it was raining last night, so I put the truck in reverse and let the clutch out, truck doesn't budge an inch and stalls.

I think I'm ok with the dual 7. my only actual issue is that it seems like I have TOO much assist, in that my pedal gets really soft once the truck is running. maybe I just need to get used to the new booster