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What mast. cyl. bore to use.

1464 Views 9 Replies 3 Participants Last post by  murrsue
Classic performance sells a kit to convert my 57 chev truck to a modern booster and master cylinder that mounts in the original location under the floor. (I have upgraded to disc/drum.) I'm going to build the brackets myself and I've got a fox body Mustang booster. Now I have to buy the Corvette master cylinder as pictured. It comes with 1 1/8" or 1" bore and I'm not sure which one to get. (Of course they won't tell me) Is it the larger bore that requires less pedal force or vice versa? The original pedal set-up only has 2" from the pedal pivot point to where the booster rod hooks up so I'm thinking there's a lot of leverage there already so I will require less force from the m/c. Thoughts??

p.s. see pic at


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Large bore gives more volume, less pressure

Smaller bore gives less volume, but more pressure.

Too large, and there wont be much pedal travel, but it will be too hard to press.

Too small, and the pedal will travel too far to have effective brakes.
If you're using the manual brake pedal ratio with your power booster, the larger master should be fine.

If you know your pedal ratio and front caliper piston area, I could tell you more accurately which size, though it also can depend on booster strength, rotor diameter, and pad CF.
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Okay, I'll go with the 1 1/8" bore. Another post said the corvette never came with disc/drum though. Is that true? The kit comes with a corvette master cylinder.
Take a look at Master Power Brakes web site, a wealth of information there.

Yeah, Corvettes have been disc/disc for a long time. Youll want a residual valve for the rear drums. They also might have a deep hole master cyl, so make sure your booster pushrod matches. The 1 1/8 was for P/B and the 1" was for manual brakes.

GM had a lot of disc/drum 1 1/8" masters. Almost all of their P/B passenger car applications and 1/2 ton trucks were 1 1/8" disc/drum thru the 70s. Full size and 1/2 tons thru the 80s also, though many were aluminum step-bore.

Some masters were deep hole and some were shallow... most of the P/B cars were shallow hole 1 1/8". Some of the manual applications also got a 1 1/8", but most M/B had 1".
Yesgo, thank-you, thank-you, thank-you. I love this site! Great info. That info from mpbrakes is great. I figured I have a 5.5:1 pedal ratio, so a 1 1/8" master cyl. would give me 415 lbs of psi output. Is that okay?

I still can't figure out why 3 aftermarket kits for this under floor kit use a corvette m/c if they are disc/disc.

When you say shallow/deep mast.cyl. are you talking about compatability with the booster? This 87-93 mustang 5.0, 7" diameter booser has an adjustable input pin. It's 2" from the bottom of the hole to the end of the pin or 1 1/4" from the bottom of the studs. The bore of the hole where the master goes in is 1 5/8". Any idea if this is deep or shallow?

thanks again, Murray
Yes I mean the hole in the piston where the booster pin goes. I dunno if that is deep or shallow. I never measured. Sounds deep though... or in between.

This Camaro site has some good pics of the differences on the 2 masters and GM booster pushrods:

I think the 'Vette masters are both deep hole, though I'm not positive on the P/B master. That could be one reason a lot of companies use them. They also have decent volume distribution and cylinder stroke.

I think Baer makes pushrod extenders if yours wont extend that far. Did you figure out how to get the pedal pushrod to work on your truck with that booster?

Also, how much line pressure you need really depends on what brakes you have at the wheels. The overall ratio (cylinder ratio x pedal ratio) and the booster are really what's important.

With that pedal ratio, 2 15/16 single piston calipers, and the tiny Fox body booster, you may be just right with the 1.125" master. It really depends mostly on the size of the caliper pistons and the effectiveness of the booster. You may be happier with a single diaphragm booster at that pedal ratio, or maybe a larger master.... I still don't know what front brakes you have. I'm just assuming the common 70's GM single piston.

Of course, even the big car companies have bulletins and recalls to change master cylinder sizes, booster diameters, friction compounds, or balance valving occasionally. No matter what you choose, it is an experiment the first time, but math and common sense can get you in the ballpark, and perhaps perfect the first time.

Dont put too much faith in the Q and A section of MP, though it is good as a general guide for most (but not all) GM midsize, fullsize, and 1/2 ton, a lot of their A's are inaccurate or dead wrong for many Ford and MOPAR, as well as some GM.
(I'll cite one example. 1973 Dart master cyls. 1 1/32" manual disc, 15/16" power.) There are others, and other A's I could crush, with specific examples and reasons for them, but it is okay as a guide for most people. Mostly all they really need to do is add 'usually', or 'often' to more of their answers. I guess everyone knows there are always exceptions. Sometimes I just go off. Sorry. :end rant: ;)
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I got the pedal pushrod linkage done last night. It's similar to what the aftermarket kits are using. It's adjustable for pedal hieght. I made the connection at the booster end solid and it swivels at the pedal end. The way the fox body booster linkage droops downward I was afraid the push of the linkage wouldn't be nice and straight so that's why the connection at the booster side is solid. I painted up the metal brackets last night so I'll assemble it tonight. The GM 70's 1 1/8" bore master cylinders are very plentiful but they look pretty tall in the pictures I saw in on-line catologs. I'll do some measuring before I buy one but it should fit. The corvette m/c is nice and low but I'm still confused how they can be used for disc/drum.

I put a 78 Chrysler Diplomat torsion bar front ifs clip in. I'm not sure what size the pistons are in the calipers. The rear has a drum 8.8 out of a Ranger 4x4 with 3:73 gears.
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A disc/disc master can work on a disc/drum application if you use an inline residual valve for the drums.

A disc/disc master generally has a longer piston stroke and more volume available to the rear than a disc/drum.

The 'Vette P/B master should do you fine. The front resivoir should be for the front brakes. It should carry more fluid volume thru the line due to a larger brass orifice in the oulet port.

The Diplomat calipers should be 2 3/4" piston. That ought to put you at a firmer pedal than the ones I thought you had. Your front cylinder ratio will be 5.98:1 making your overall front ratio 32.67:1 which is still on the high side of most power brakes but in the ballpark. If the booster ever fails, you'll be thankful for that ratio.... but, if the pedal feels too mushy for you, you could move up to a 1.25" or larger master (if you can find one which fits) and still be well within limits. I'm pretty sure that booster is dual diaphragm, being as it is so small diameter... it kind of throws a fork in my thoughts.

The Ranger 8.8 generally had 10" drums, but could have 3/4 or 13/16 cylinders. I doubt you'll need much if any rear prop valve, though i could be wrong. You can adjust the rear bias somewhat by changing the cylinder size... depending on which way you need to go and what is in there. You might still want the delay (metering) valve for the fronts.

I'm glad to hear you have the pushrod at a straight line to the booster.

I guess I got kind of involved. I could've just posted "Yes. That master should work."

The 1978 G30 Van has a 1.25" disc/drum which is similar in appearance to the 'Vette master. I think the front port is for front brakes on it too, but not positive. GM generally has them set that way, opposite of Ford and Mopar.

-Good luck
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corvette m/c

I stopped at the parts store after work. Looked at a 75 Camaro m/c. It uses the same m/c as many 70's mid to large chevys. The m/c is pretty tall. It is also a shallow input end and won't work with the Fox body Mustang 5.0 booster.
I told him about the corvette m/c. He had a '73 1/18" bore in stock and we took a look. It has a deep end and it is shorter in hieght. The bowls are not equal as I thought they would be for a disc/disc. I guess it makes sense that the rear disc would need a lot less stopping power than the front, thus the smaller bowl. Now I see why those kits use the corvette master cylinder for disc/drum applications. I brought one home to check it out. I'll order a residual valve for the rear drums if this master works out.
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