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I want to get rid of the brake booster and convert to manual brakes. I know I will need to replace the master cylinder with one of the correct size. But I'm unsure of how to choose the correct master. Also, I would like to choose one to where I don't have to change the pedal ratio if possible.

How would one figure out what size master cylinder is needed to go from assisted no manual brakes?

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My car has the master from a late model VW Beetle. Discs up front and drums in rear. Floor mounted pedal and remote reservoir mounted to upper chassis tube. Have a Willwood proportioning valve to adjust bias. Works great, many track days and auto-x events on it with no issues.

I used the RX7 master cylinder and all of its calipers. I deleted the power boost module and went manual. I did make my own pedal assemblies at ~ 6:1 (or maybe 6.5:1) I also used the donor's proportioning valve. I like them as they are. I'm sure it has more to do with the "system" rather than just the master. Also remember we are stopping a car that is about 1/2 the weight of the donor so pedal pressure is not as high as when in the donor vehicle. The easiest thing to do is to just remove the booster and try it.

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Chuck.

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Brake boosters have a built in ratio just like the pedal ratio. I think my Miata one is about 6:1. If you removed the booster it should feel about 6x stiffer in that case. Adjusting the pedal ratio can offset that a little. Changing the bore of a master cylinder is similar, a larger master cylinder pushes more fluid per inch of movement so it is firmer.

If you half the cross sectional area of the Master cylinder you should take half the pressure to move. However you need to push the pedal twice as far to move the same amount of fluid.

Why do you want to change to Manual brakes? I know some people prefer the feel as a booster has a slight delay. I love my booster though and actually like the delay as it help reduce tire lock up.

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My set up:

Master Cylinder - 1-1/16
Brake booster - 7:1
Pedal ratio - 3.4:1 or somewhere close to this

Is there a formula or a way to find ideal MC size with this information?

I want to delete the booster for a few reasons. The main reason being the space it occupies. Secondly I'm a sucker for weight loss. And 3rd, but could be 1st.... it's ugly!

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How hard do you have to push the brake pedal with the current boosted setup to lock the brakes? Your current setup will give you 2109 psi for 100lbs of pedal pressure, just as an example. So if you eliminate the 7:1 booster, you will need 700lbs on the brake pedal to get the same line pressure.

Piston area (3.14 x radius squared) x pedal pressure x pedal ratio

This will give you the line pressure for any given combination of master cylinder size and pedal pressure without the booster. Your 1.0625 master cylinder has an area of .886" square. To cut the pedal effort in half without any other changes, you need half the piston area in the master cylinder, so 0.75" diameter. This will also double the distance you need to push the pedal for the same amount of braking.
Kristian

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Your question, as stated, is unanswerable. It all depends on how hard a pedal you can tolerate. The smaller the MC diameter, the softer the pedal and the longer the pedal travel. As you decrease pedal effort, you will reach a point there is not enough travel and the MC does not move enough fluid to fully apply the brakes. Where that point lies depends upon your system.

Bill

Good information, I realize now that I need to do a lot more reading to understand this properly.

Right now it looks as if a 7/8 master cylinder with adjusting the pedal ratio will be the ticket. But I can't really make a this decision yet as I am not fully educated yet. I will now dive deep into the Google looking for useful information.

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Plans: Single Rotor turbo sub 1000lb machine!

I would prefer the feel of the pedal to not change going from assisted to manul. Or as close to that as possible that is.

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Plans: Single Rotor turbo sub 1000lb machine!

Another question on this topic when choosing a master. I was looking at A the Tilton 75-875 compact master. I can't tell from the generic pictures I keep finding and the descriptions do not say either. But will these have one port or two that lead to the proportioning valve?

If it's one, can I split (Y) it off to go to either side of my prop. valve? The only thing is, with one port, if something fails I will lose all breaks instead of just one side. Correct?

For reference, I attached the prop. valve I am using.

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Plans: Single Rotor turbo sub 1000lb machine!

Imho, there is a lot of misleading information about the standard “vacuum suspended” brake booster. It is not a ratio multiplier of the pedal effort like a pedal is, it just seems to be within the first 50 lbs of pedal force. Basically, the assist is primarily the result of the ratio of the highest engine vacuum before pedal application to atmospheric pressure (14.7 psi). The diaphragm is between the two. Because the difference in pressure is so small, the diaphragm must be big for greater area or use more than one.

The amount of assist is fixed when you first apply the pedal, and you get it all by the time you’ve applied about 50 lbs of force. Then the pressure increase curve parallels the non-assisted/manual brake curve.

Assisted vehicles usually have lower pedal ratios by about a 3:1 difference so the brakes do not lock up too easily (without abs) and there is better feel.

I’d remove the booster given the reduction in curb weight and use the original pedal ratio and master, but consider leaving yourself the ability to increase the pedal ratio later by either altering the pivot point for the pedal or raising/lowering the master cylinder on the firewall, toward the pedal pivot.

Use a dual circuit master. Then you should notice a lack of braking ability or locking up but not stopping like it should when you lose a circuit, versus the possibility of zero warning and/or slowing.

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Yeah, I wouldn't use a single port master cyl for the reasons that MV8 said. Use the original master as is or go with the 2 masters with balancing bar. I guess you could just block off the vacuum port and see how the feel is.

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Chuck.

“Any suspension will work if you don’t let it.” - Colin Chapman

Visit my ongoing MGB Rustoration log: over HERE

Or my Wankel powered Locost log : over HERE

And don't forget my Cushman Truckster resto Locostusa.com/forums/viewtopic.php?f=36&t=17766

I've been using this spreadsheet to design my system and make my component selections.

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-Justin

"Orville Wright did not have a pilots license." - Gordon MacKenzie

That thing is awesome!

But towards the very end of it I am unsure if I am using it correctly. It has 2 locations for master cylinder. Front and rear. I will only be using one master. It said something about splitting the stroke if using a tandem master. Since the master I'm using has a 1.100" stroke, I put .550" in each cell. Is this correct?

Also, I'm unsure what the answers mean to me... the data is there. But I don't know how to apply the data to useful information.

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Plans: Single Rotor turbo sub 1000lb machine!