Master Cylinder bore size question

[Keeper]

Okay I was planning on looking for a 1 1/8 bore master cylinder, but after reading all the posts on the subject in here I am confused.

I have a disc/disc setup. Manual brakes.

I was thinking that the larger bore would push more fluid=better braking. But from what I am reading, smaller bore=more pressure=better braking.

So you can see my confusion.

Can someone explain?

 

[Corley]

It's all about the volume of fluid vs the pressure of the fluid. You are correct in that a bigger master cyl means more fluid / less pressure, smaller master cyl. means less volume, higher pressure. You've got to figure out the balance that works with your wheel cylinders / weight / pedal leverage / front to rear balance / etc. It is not really that difficult to figure out, and if you get it wrong you have options to correct it. (Different master cyl., different leverage on pedal, different bore size calipers, power assist, various valving options, etc.) Google search / read / read / read... Since everyone has a different mixture of brake parts, don't just guess, educate yourself on it, it's important stuff...

 

[Ted Brown]

It's all about the volume of fluid vs the pressure of the fluid. You are correct in that a bigger master cyl means more fluid / less pressure, smaller master cyl. means less volume, higher pressure. You've got to figure out the balance that works with your wheel cylinders / weight / pedal leverage / front to rear balance / etc. It is not really that difficult to figure out, and if you get it wrong you have options to correct it. (Different master cyl., different leverage on pedal, different bore size calipers, power assist, various valving options, etc.) Google search / read / read / read... Since everyone has a different mixture of brake parts, don't just guess, educate yourself on it, it's important stuff...

That was well said, I agree with you completely, read, read and read some more, but comprehension is mandatory.

 

[bobscogin]

Here's a good training video that explains the physics of hydraulic brakes.

Bob

 

[ellis8500]

Yes, very important stuff. I am using GM metric calipers in front, Ford SUV calipers in back. Because of different caliper piston sizes, front brakes will provide about 70% of stopping power. Using a 75 Corvette master cylinder for manual disc brakes. Figured if it was good for a 3,500 lb car, it should be really good for a 2,000 lb car. Pedal ratio is 6:1. Calulation show about 1G or more stopping power if the wheels don't lock up. I can always add a proportioning valve if I need one.

 

[Keeper]

Got to love old Army training videos Thanks.

After looking up what the cars that my parts came from used. I think 1 1/8 will do fine.

Still trying to figure out how a smaller bore makes for higher pressures at the wheels. Maybe I will track down part 2 of that video.

 

[GAB]

Keeper, a little math will give you the answer to the question.

Lets say that you are pushing on the brake pedal with 75 lbs of force and that you have a pedal ratio of 6 to 1. Multiplying those gives a force of 450 lbs of force acting on the plunger of the master cylinder. With a master cylinder that has a bore of 1-1/8" you have a piston that has an area of .994 square inches. (A = pi r[sup]2[/sup]). To get the line pressure, you divide the area of the piston by the input force. ( .994 / 450) = 452.7 lbs of line pressure. Using the same input figures but using a 7/8" bore master cylinder we get an area of .601 square inches and when that is divided by the 450 lb input it gives a line pressure of 748.8 lbs of force. The big mistake that most people make is they want to multiply instead of divide.

With disk brakes, the name of the game is line pressure. That is all that pushes the friction material out into contact with the rotor. Take a piece of material and hold it between your thumb and forefinger. Apply a light pressure on the material and pull on the material with the other hand. Now do the same thing using all of the squeeze that you can muster. That is the basic principle of disc brakes.

As far as worrying about volume of fluid, don't get too concerned about it. If you do the math on a disc/drum system like might be found on a typical bucket, you'll find that the amount of fluid that is moved is around 1/3 of a cubic inch for the whole system....that is about the size of a piece of Starburst candy....or a Walnetto candy for us old guys.:cripple:

 

[tnoftsger]

Keeper, a little math will give you the answer to the question.

Lets say that you are pushing on the brake pedal with 75 lbs of force and that you have a pedal ratio of 6 to 1. Multiplying those gives a force of 450 lbs of force acting on the plunger of the master cylinder. With a master cylinder that has a bore of 1-1/8" you have a piston that has an area of .994 square inches. (A = pi r[sup]2[/sup]). To get the line pressure, you divide the area of the piston by the input force. ( .994 / 450) = 452.7 lbs of line pressure. Using the same input figures but using a 7/8" bore master cylinder we get an area of .601 square inches and when that is divided by the 450 lb input it gives a line pressure of 748.8 lbs of force. The big mistake that most people make is they want to multiply instead of divide.

With disk brakes, the name of the game is line pressure. That is all that pushes the friction material out into contact with the rotor. Take a piece of material and hold it between your thumb and forefinger. Apply a light pressure on the material and pull on the material with the other hand. Now do the same thing using all of the squeeze that you can muster. That is the basic principle of disc brakes.

As far as worrying about volume of fluid, don't get too concerned about it. If you do the math on a disc/drum system like might be found on a typical bucket, you'll find that the amount of fluid that is moved is around 1/3 of a cubic inch for the whole system....that is about the size of a piece of Starburst candy....or a Walnetto candy for us old guys.:cripple:

Beware, too small a Master cylinder can be done. I did it, started with a 3/4" bore and couldn't generate enough line pressure with early GM calipers. Did the math and discovered i was only moving the caliper pistons about 0.03". Not enough, upgraded to 7/8" MC and problem solved. 0.05" movement.

 

[bobscogin]

Beware, too small a Master cylinder can be done. I did it, started with a 3/4" bore and couldn't generate enough line pressure with early GM calipers. Did the math and discovered i was only moving the caliper pistons about 0.03". Not enough, upgraded to 7/8" MC and problem solved. 0.05" movement.

With the 3/4" master cylinder it's volume you were lacking, thus pressure couldn't build before the piston ran out of travel. When choosing a master cylinder, remember that the bore to area relationship is non-linear. When you went from a 3/4" to a 7/8" master cylinder, the bore was 17% larger, but the 7/8" mc has approx 37% more area.

Bob

 

[tnoftsger]

Yep, you got it. Shoulda done the math before i bought it. Live and learn

 

[Keeper]

Thanks, I was looking at it as "If I can push more fluid, I can build pressure faster" just seems to make more sense to me.

Maybe I will look at a 1 inch instead.

I have the metric GM calipers on the front, and the jag calipers on the rear.

 

[bentbaggerlen]

My bucket started with the Mustang master cylinder supplied by Spirit, after checking I found that its a disk/drum master cylinder. The brakes are OK, but could be better. I was thinking of adding a 7" booster. But now I'm thinking that I should replace the master cylinder first. Can any one tell me what master cylinder I could use as replacement for a disk/disk set up?

 

[ellis8500]

For a disc/disc system you can use a '67 - '76 Corvette manual brake master cyclinder. These are designed for 4-wheel, manual disc brakes. These have a 1" bore. Can get them for $30 on up depending whether new or rebuilt. Check all the popular auto parts store in your area.

 

[Keeper]

For a disc/disc system you can use a '67 - '76 Corvette manual brake master cyclinder. These are designed for 4-wheel, manual disc brakes. These have a 1" bore. Can get them for $30 on up depending whether new or rebuilt. Check all the popular auto parts store in your area.

Thanks I will look for one of those.

 

[bentbaggerlen]

Thanks Mr. Bill!
I went looking on-line and found

Advanced Auto-
Part No. MC64980 about $100.00 brand new
Part No. 13-1371 about $50.00 bucks for a "Seclect" rebuilt
Part No. 10-1371 about $35.00 bucks including $10.00 core charge for a rebuilt.

NAPA -
Part No UP 36367 Brand new "NAPA ULTRA PREMIUM " $50.00 bucks. Guess I'm calling NAPA in the morning.
Part No TS 101371 rebuilt $45.00
Part No UP 472 Rebuild kit for the master cylinder, $33.00... Hardly seams to be worth the trouble when a complete rebuilt M/C is $24.00 with a core.