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Old 03-31-2009, 06:05 PM
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Settle a Brake Issue

I have a 38 Chevy Streetrod with a brake issue. It has a 327 with a little cam and the vacuum is marginal to the power booster. It has a 7in dual diaphram booster and a vacuum tank but brake feel is poor and it takes a lot of leg to hard stop. I am proposing a change in master cylinder piston size to increase leverage? My mechanic says the ratio of piston size to the calipers isn't an issue.

If you agree that the piston in the master cylinder has a role in creating leverage, a smaller piston bore would create more leverage? or larger?

I formerly ran a vaccum pump that quit. it was the original GM pump from the 70s. It vibrated. I had a new pump donated that was noisy as hell and it was an embarresment. I want to run without a pump.

Thanks for the help.

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Old 03-31-2009, 06:28 PM's Avatar
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A smaller master cylinder piston will give you more braking force.

My own ride had a "Stand on em" to stop, non power system. I went down two sizes in the master cyl department and now the thing feels like power brakes. Stops Great!!
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Old 03-31-2009, 07:12 PM
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I do agree a smaller MC piston would help but what type of MC is on it? I am guessing it has a Corvette style MC on it although there are better ones out there. I like the Chrysler style/Strange aluminum unit, they are the MC of choice in the dragrace community but they have not caught on in the streetrod world yet.

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Old 03-31-2009, 08:20 PM
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Your mechanic is incorrect.
Master cylinder size does make a great deal of difference in how much line pressure you can develop with a given pedal effort. It is simple hydraulics.

here are some numbers to ponder..with a 6:1 pedal ratio and 75 pounds of force on the pedal you will generate the following line pressures with various bore sizes..

1 1/8" 453 psi
1" 573 psi
7/8" 748 psi

with a 5:1 pedal ratio and 75 pounds of force on the pedal the pressures are roughly 80% less..

But you need to be careful, because although brakes need pressure to work, and depending on the sizes of the bores in the calipers and wheel cylinders, they also need a certain volume of fluid to get the pads fully seated against the disc or drum they act upon.. Some disc brake pistons retract more than others , and you'd be surprised how much volume and pedal travel is used up just getting the pads against the discs.

But a rule of thumb can apply here, if your pedal is currently high and hard, you can go down a size or 2 in bore and still be able to deliver enough volume with a higher pressure to the brakes and they will still operate.

There are alot of variables here, such as; you can only get so much pushrod travel out of an underfloor brake pedal and master cylinder, due to the shape and dimensions of the pedal. Those aftermarket pedals are usually about 6:1 ratio, which will give you about 1" of pushrod travel for 6" of pedal travel. (motion ratios are interchangable with force ratios in this case). IMO, you really don't want to operate a pedal with less than 1" of available pushrod travel.

The type of master cylinder will do nothing to change your pedal feel, it is all in the bore size and pedal ratio, and these need to be compatable with your caliper piston size and wheel cylinder bores. The aluminum chrysler master cylinder is a good unit for remote reservoir work, but IIRC it is only available in a 1 1/8" bore size. The late mustang units are similar to the chrysler.

Usually a 1 1/8 bore master with a 6:1 pedal and any kind of booster , used with a set of GM metric calipers and a rear drum with 15/16 bore wheel cylinders will be able to be swapped out with a 1" bore and still deliver enough fluid to keep everything working, you'll wind up with a pretty low pedal, but you will have a better brake feel, as long as you don't have a set of low drag calipers, or calipers that retract the piston more than .010" upon brake release..Some of the metric calipers will allow about .020" of retraction, and act like the low drag calipers...

If you are running any of the aftermarket 2 or 4 piston calipers, like Baer, or wilwood, those have little baby pistons and require a 15/16 bore master cylinder.

check out some of these pages from the master power brake website and you'll see some more info about your brake system compatabilities..

here is one about low drag calipers.

here is a link to the page with those and even more cool reading material about brakes. The stuff is pretty much right on, but I've found some of the vacuum booster numbers are exagerrated..

Later, mikey
my signature lines...not really directed at anyone in particular..

BE different....ACT normal.

No one is completely useless..They can always be used as a bad example
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Old 03-31-2009, 09:01 PM
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Corvette master cylinders come in both power and manual. The power has the bigger 1 1/8" piston and the manual a 7/8 or 1 inch. I'm actually running a manual MC with a 7 inch booster. The difference is in the brake pedal push rod also. The power assist has a longer rod and the manual a shorter rod. If that helps.
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Old 04-01-2009, 05:44 PM
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Problem Solved!

The mechanic rigged a undercar vacuum tank at the back of the car and although it doesn't create vacuum, it delays the low vacuum and evens out the vacuum to the booster. Good feel and it will stand on end if you need it. I couldn't be happer to get rid of the vibration and noise of a pump.

Happyness is a good running streetrod with three car shows this weekend!

Thanks for all the good information, Chuck
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Old 07-31-2014, 09:12 AM
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Another car --same problem

I'll have my buddy look at the smaller bore MC. Is it possible to build a booster with a more flexable diaphragm??
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