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Hot Rod Forum : Hotrodders Bulletin Board > Tech Help> Suspension - Brakes - Steering> 7 or 8 inch booster
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Topic Review (Newest First)
02-17-2008 08:23 PM
snkbyt If the master cylinder is below the wheel cyliders or caliphers then you use a 10# RPV inline to drum brakes and 2# to disc brakes. This keeps them from sucking air around the seals or pucks as they retract and air entering the system. If the master is above the wheels then you don't need RPvalves. A disc drum master already has a RPV in the rear drum exit. Under the tapered brass seat where the line connects. You can check to see if there is one in the master by carefully inserting a paper clip into the master cly. brass line seat and feel for a rubber valve..
On a streetrod I always use the inline adjustable proportining valve, like Wildwood sells, in the rear line and I still think I could get away with not usining one on Disc-disc.
On my truck using the Fiesta booster I mounted it back on the trans. crossover after welding on a bracket then used the original pedal with a 2' extension to the booster. It's called streetrodding or IE; make it work. LOL
02-17-2008 12:23 AM
powerrodsmike Yes it is a good idea to install RPV's.

A prop valve is good to have as well, Speedway sells 2 brands of them for cheap.

02-17-2008 12:19 AM
71corvette is a residual pressure check valve needed on a frame mount master cyl? also what would be a good porportioning valve for this setup?
02-16-2008 11:41 PM
powerrodsmike Bigger boost is better with discs, But if you are concerned about clearance go with a dual diaphram 7".
An 8" single has about 50 sq inches of diaphram area, a 7" has about 38 sq inches and a dual 7" has about 70. sq in.

Each inch of vacuum is good for about 1/2 psi, so you will get about 600 pounds of boost at the mc piston from a 7" dual ...450 with a 8" and 340 with a 7" all at 18" of vacuum.

An 8" dual will give you about 850 #.

The 8" single should be alright if you don't mind pushing the brake a little harder. If you are using the TCI pedal assembly for that truck, then it is a high ratio arm anyway, and your brakes will be easy enough...(about 6 or 7 to 1 IIRC.)

I've installed the boosters with only a couple of inches clearance with no problems.

Edit..I didn't see the post above where you said you were considering dual diaphrams.

If you are using GM calipers use the 1 1/8" master.

Hope this helps,
02-16-2008 11:39 PM
Originally Posted by snkbyt
I used a single diaphram 7-8"booster from a Ford Fiesta and it works Okay. It could be better on my 52 Chev PU. If you get a dual diaphram either size will work good. I didn't have any problem with header clearance.
yes it is a dual diaphram
02-16-2008 11:28 PM
snkbyt I used a single diaphram 7-8"booster from a Ford Fiesta and it works Okay. It could be better on my 52 Chev PU. If you get a dual diaphram either size will work good. I didn't have any problem with header clearance.
02-16-2008 10:32 PM
7 or 8 inch booster

I am mounting a frame mounted booster on my 1950 chevy truck. TCI offers a 7 and a 8 inch version I am not sure how much clearance I will have with headers, has anyone installed the 8 inch booster? I will have 11 inch disc up front and the drums on my ford 9 inch in the rear. how much better would the 8 inch booster be on a 3200 pound truck?

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