What Bore Master Cylinder Do I Need?

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805moparkid

805moparkid

Slant and AFX Guy
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So after figuring out that most all the boosters have the same "spread" I am looking at a GM dual 7" booster with correct aluminum master. Now where I get stuck is the bore.

Currently running stock LBP 73+ discs and 10" drums. MY plan shortly is to go to the B Body 11.75 rotors with Wilwood calipers, with the jeep rear discs, with 18x9's with 255/40-18's on 4 corners, basically protouring.

I was looking at a 1.25 bore as i'll have half as much surface area on the new booster (75.6) than the stock single 8" (50.2).

Thoughts?
 
Call the Ram Man. He's an expert on Mopar brakes and can hook you up with the right part for your application. Tell him what you got and let him recommend the proper master cylinder to you. He restores them and sells them. He's very helpful and a nice guy.

http://www.theramman.com/

7200 Winters St
Fort Worth, TX 76120
Telephone: 817.429.0105
 
I have the 11.75 fronts (stock 2.75" calipers) and 10x2.5" rear and manual brakes with a 1 1/32 master cylinder. Hard to lock them up...so I will switch to a 15/16" master cylinder, which will make the pedal force lower, but also increase pedal travel.

A stock mopar with power brakes has a 15/16" master cylinder. If you think the pedal travel is too long, then go up a little at a time. For your situation, the 1 1/32" is probably a good place to start.

IIRC, a dakota has a 2-bolt 15/16" master cylinder, and the diplomat (M-body) has a 1 1/32". The pickups have the big ones.
 
goldduster318 said:
I have the 11.75 fronts (stock 2.75" calipers) and 10x2.5" rear and manual brakes with a 1 1/32 master cylinder. Hard to lock them up...so I will switch to a 15/16" master cylinder, which will make the pedal force lower, but also increase pedal travel.

A stock mopar with power brakes has a 15/16" master cylinder. If you think the pedal travel is too long, then go up a little at a time. For your situation, the 1 1/32" is probably a good place to start.

IIRC, a dakota has a 2-bolt 15/16" master cylinder, and the diplomat (M-body) has a 1 1/32". The pickups have the big ones.
Click to expand...

ok so maybe i should be looking for the 1 1/8" bore first and see how i like it? do you think the 1.250 would be to hard even with the dual 7" or dual 8" booster and stock/high vacuum? (14-20)
 
805moparkid said:
ok so maybe i should be looking for the 1 1/8" bore first and see how i like it? do you think the 1.250 would be to hard even with the dual 7" or dual 8" booster and stock/high vacuum? (14-20)
Click to expand...

The larger the bore the more fluid it moves per given stroke, and consequently the less pressure it produces. Smaller bore M/C's have more pedal travel but pressurize the brakes harder to give better pedal feel.
 
fishy68 said:
The larger the bore the more fluid it moves per given stroke, and consequently the less pressure it produces. Smaller bore M/C's have more pedal travel but pressurize the brakes harder to give better pedal feel.
Click to expand...

Huh? Move more volume thru the same path (brake line) is going to cause higher pressure. The smaller bore has more travel because it takes longer to get same volume thru brake system under less pressure.
 
805moparkid said:
Huh? Move more volume thru the same path (brake line) is going to cause higher pressure. The smaller bore has more travel because it takes longer to get same volume thru brake system under less pressure.
Click to expand...

That's true but that isn't what I was getting at and never said anything about moving more fluid through a given line size. I was just giving examples of what happens with big vs. small bore size giving the same pedal effort. Moving more fluid through the same line will cause higher pressure but it takes more pedal effort to build that pressure.
 
I'd give Bill at RMS a call. He set me up with the correct master for my 4 wheel disc setup quick and easy. Price tag wasn't bad either.
 
I just finished that conversion and used the stock Mopar disc/drum master cylinder. The pedal travel is longer but the pressure it takes from your foot to engage them is good. Not to hard not to soft.

I have the larger bore in my fat fish and it takes more effort from my foot which I don't really like.....hard on the knees after a long day of driving.
 
fishy68 said:
That's true but that isn't what I was getting at and never said anything about moving more fluid through a given line size. I was just giving examples of what happens with big vs. small bore size giving the same pedal effort. Moving more fluid through the same line will cause higher pressure but it takes more pedal effort to build that pressure.
Click to expand...

ok then we are on the same page. Im not worried about the effort as I decided to run the dual 8" booster as the 7" wasn't going to gain me where I was hoping.

I ordered a dual 8" booster and 1.250 master so i'll get it installed when I get a chance and let you guys know how it goes. They seem a common upgrade for chevy's with factory mushy pedals so im not to worried.

will update
 
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