Master Cylinder bore size= what is the criteria?

[WagonCrazy]

Abstract question: What is the criteria for determining the proper bore size for a master cylinder purchase?

Specific reference: I'm needing to decide on a new MC for my Nomad build. It has one of ChevyNut's C4 frames and the front and rear suspension from a 96 Corvette. Stock calipers and rotors all the way around. Will be plumbing new brake lines, and havent decided on line size to use for that either. (1/8 inch or 3/16 inch)

I'm running a Hydroboost unit for the power brake portion of this equation.

Should I get a 1 1/8", 1 inch or 7/8 inch bore master cylinder? And why...

I'm thinking I should just use a MC with the same bore size as what the 96 C4 corvettes used, as logic tells me that would result in a "matched system" as GM had originally designed it, but don't know what that bore size is...

[WagonCrazy]

After posting this, I read Summit's Q&A site on the subject. here's what it says...

Q: What is the difference between a disc brake and a drum brake master cylinder?
A: Disc brakes require more fluid to operate, so a disc brake master cylinder holds a larger volume of fluid than a drum brake master cylinder. A four-wheel drum brake cylinder has a residual pressure valve to maintain a small amount of pressure on the brakes. This prevents air from entering the braking system. Using a drum brake master cylinder with disc brakes reduces braking effectiveness due to decreased fluid volume and rotor “drag” caused by the pressure retained by the residual valve.


Q: What is the difference between a manual brake and a power brake master cylinder?
A: Manual brake master cylinders have a smaller piston bore diameter to maintain sufficient fluid pressure in the braking system. They also have a deeper hole for the brake pedal pushrod. You can use a manual brake cylinder on a power brake system, but not vice-versa; the power brake cylinder’s larger piston bore cannot supply the required level of hydraulic pressure required by manual brakes.


Q: Can I use a master cylinder from a disc/drum brake system with a four-wheel disc brake system?
A: No. A master cylinder designed for four-wheel disc brakes is required to supply the proper fluid pressure and volume.


Q: What is the proper master cylinder bore size for manual brakes?
A: The bore should be no larger than one inch to maintain the proper level of hydraulic pressure.

[56-210Sedan]

Remember what ever size you get for master cylinder, you will need for caliper

[Rick_L]

Gee you've asked a bunch of questions but they are good ones.

In general you should be matching your master cylinder bore to the bore size of the calipers. The C4 calipers have a relatively small bore size, so a relatively smally bore master cylinder matches up. You won't go wrong using the same bore size that was used on a stock C4. I think that's in the neighborhood of 7/8" for a manual setup and 1" for power (Chevynut will correct me if I'm wrong). With a hydroboost you may be able to run a slightly bigger m/c than was stock on C4s with power brakes.

Remember this: a smaller master cylinder will create more pressure at the calipers, but the pedal stroke will be longer.

Because the C4 calipers are smaller than the typical cast iron GM calipers used on most trifives, the master cylinder size is smaller. The cast iron single piston GM calipers (either the Chevelle type or the metric type) usually need a 1" bore master cylinder for manual and a 1-1/8" bore for power.

The recommendations for disc vs. drum are based on the fact that disc brake systems don't have adjustment. The fluid level drops as the pads wear. You can use small reservoir master cylinders with discs but you have to check the fluid level and top it off as needed.

[Rick_L]

Did a little bit of research, apparently the C4 bore size is 7/8". I also wonder if they even offered manual brakes.

[chevynut]

Paul there are more factors than you listed to design a brake system. You need to consider pedal ratio, master cylinder bore size, rotor diameter, swept area, and caliper bore size.....and whether you use power brakes or not, and what kind of booster you use. So it can get complicated real fast. I don't know how a hydroboost compares to a vacuum booster as far as boost ratio, but I know that my truck's hydroboost is amazingly good.

Also keep in mind that part of this is what "feel" you want.

That said, the late C4 Corvettes used .87" (88-91) and .93" (92-96) bore sizes. The pedal ratio was 3.5:1 for 88-91, and 4:1 for 92-96. The line pressure at 100 lb pedal load was 1250 PSI front and 750 PSI rear for 88-91 and 1160 PSI front and 680 PSI rear for 92-96.

If you want the same brake "feel" as the C4 Corvette you need to shoot for those same pressures with the same pedal force.

The tri5 pedal ratio is 6:1 so you already have a pressure advantage. However, you always trade off pedal travel for pressure when you change master cylinder bore size. As MC bore decreases, pressure increases but pedal travel increases because you have to displace the same amount of fluid to the calipers.

Let's assume your hydroboost has the same boost characteristics as the corvette booster. With a 7/8" bore MC and a 4:1 pedal ratio, your brakes would feel about the same as a C4 Corvette. Since your actual pedal ratio is 6:1, your pedal will travel 1.5 times as far but you will get more pressure with the same pedal force. To counteract the excessive travel, you would want a bore area 1.5 times as large, which would be 1.07". So a 1 1/8" bore would give you a little less travel than a C4 Corvette, and about the same pressure.

Go with 3/16" lines instead of 1/8". I think it will be easier to find fittings. I went 1/4" everywhere, mostly because I used all AN stainless fittings and I already had a bunch of -4AN stainless fittings. Line size really doesn't matter unless you get too small such that it restricts flow.

[WagonCrazy]

Thanks a bunch for that guys. Next question:

As long as I get a disk/disk master cylinder, do I still need a separate proportioning valve plumbed into the lines? Or can I assume the MC already is set for proper front and rear pressures as it was made? (My hunch is that I need to build some front and rear line pressure adjustibility into the system just aft of the MC).

If I need a proportioning valve, can I use one on the rear line only? One with a manual adjustment knob...like a wildwood style unit? How have you guys dealt with this on your builds?

[chevynut]

Paul, yes you still need a proportioning valve. I used a Wilwood one in my rear brake line.

[Rick_L]

Cnut filled in some blanks I didn't post about.

But one more thing. There are several different hydroboost units out there, the major difference is the piston size, which will directly affect the amount of boost. I think you can go slightly larger on the m/c bore than with a vacuum booster and still be good. I have a Ford p/u unit here that has a 1-1/4" master cylinder on it. But I'm assuming it has some big bore calipers too. I'm certain it would have more boost than the Astro van unit I'm going to use.

A good while back I found a listing on the net of all the piston sizes on hydroboosts, as well as a lot of OEM applications and p/n's. Most of the h/b units have a color code on the accumulator and that's a first step at id'ing them. But I didn't copy it and I haven't been able to find it again. I do know that the Astro van unit has a gold accumulator. It's one of the smaller bore sizes but it's also probably the best match for conversions on our cars.

[WagonCrazy]

DSCN2348.jpg DSCN2347.jpg

Havent a clue what this Hydroboost came off of before it was built, but there are a few numbers cast into the body and a number imprinted into the accumulator. I had asked the guy when I bought it to give me one with a silver accumulator, so not sure this number is going to be relevant.

I'm thinking I'll go with a 1 1/8" bore size MC, based on CNut's logic above.

Just not sure I like the looks of any of them. I've decided to keep the components in the engine bay "simple" and no vendor logos, names, etc. Black hose and black AN fittings, etc. Nothing polished or fancy. Just keep the primarily silver = aluminum look consistent throughout the compartment.

With that said, every aftermarket MC manufactuer puts their name cast into their products.

Still deciding what to do here.

[chevynut]

Paul, did you make that mounting bracket? I am thinking about offering them for sale...a little different than yours. The bolts on mine face backwards for a clean look. The stock bolts would have to be taken off the brake bracket. I may offer a version with just holes too.

[WagonCrazy]

No...it came withe Hydroboost unit being sold as "new" from a guy on eBay. Here's the listing of the current ones...

http://www.ebay.com/itm/NEW-55-57-Chevy-Hydroboost-and-mount-/300670492567?pt=Motors_Car_Truck_Parts_Accessories&hash=item46015b9f97&vxp=mtr

Now that I've seen yours...I like the way it looks hiding the bolts completely. much cleaner...I'd think you'd be able to see a few of those to guys who already have the Hydroboost unit.

In my build, there's a 2nd custom made bracket that uses the outer 2 brake bracket bolt mounting points. this bracket is for the 2002 Camaro clutch MC to mount "at the proper angle". So I'd have to incorporate that somehow...