The master cylinder on my'80 is leaking brake fluid all over. I replaced with a rebuilt mc 5 years ago (trying to save $$ as a new C3 owner). I am going to replaced with a new mc this time.

As I was shopping around, I found new ones come with either 1" or 1 1/8" bore. I think the stock master cylinder is 1 1/8".

What difference does 1/8" of bore diameter make in actual braking?

PS., I am replacing the booster while I'm at it with another 9" dual diaphram, so boost will be same/better than what I have now.

 

A manual master cylinder is 1 inch to build slightly higher pressure for a given leg pressure but you have to move the pedal futher to move a certain volumn of fluid.
The 1 1/8th is for power assisted brakes and you have slightly higher leg pressure but the power assist takes care of this and at the same time you need less pedal travel to pump a larger volumn of fluid.
If you run power assist get the proper master cylinder. The 1 1/8th

 

I think the diference is one is for power the other for manual brakes. One thing I do know is a bigger bore will be harder to push on the pedal, so I believe that would mean the larger bore is for power brakes.

 

Of course with hydroboost, you could run a 2" bore.....and really stop that car.....

the M Cyl may be a tad LARGE and overweight though.....

 

I run the manual 1 inch bore master cylinder with my hydroboost and feel maybe that is the reason I have problems with 100 mph braking while turning and having the rearend come around.
The combination of this bore, hydroboost and 14 inch rotors on the front might be causing overbraking on my part.

 

I'm confused, lets say a 1" bore MC has an effective area of .785 and a 1.125 MC has an area of .994. Wouldn't it take less pedal force to generate the same pressure if you have a larger bore? Not the opposite?

 

The resistance you feel, or pedal pressure isn't dependent on the size of the bore, but the amount of fluid you are trying to force through the small openings at the mouth of the brake lines. More fluid going through the same place makes it harder to squeeze it through.

Picture a shopping mall the day after Thanksgiving. One door, a few hundred people all trying to get in first to get the good deals. Who will get through faster and easier? 100 people or 120 people?

 

I think the difference is that you are talking about flow and I'm talking about pressure. A larger bore will give you more pressure out for the same effort.

 

You just answered your own question.

 

a smaller dia. piston will displace less fluid for a given piston travel thus reducing the leverage (pedal pressure) required, but, more travel is required to compensate for this as the amount of fluid reqiured for actuation of the calipers remains the same.
In other words, With a given amount of pedal pressure a smaller diameter master cylinder will apply more stopping power than a larger one but you will have to push the pedal further.

 

That makes sense based on my understanding of the physics of hydaulics. So, a 1" bore mc with a stock 9" dual diaphram booster will be more sensitive to the foot but brake pedal travel will be farther because a 1" hydraulic piston takes more travel to push the same volume of fluid as a 1 1/8" piston.

What do you think? Will a 1" bore mc with a 9" dual diaphram booster be too sensitive for practical use? Will a 9" dual diaphram booster have enough travel to push enough fluid to the calipers to stop?

 

No, it is the opposite; Pressure = Force/Area, therefore for the same given force, the smaller bore master (smaller area) will output more line pressure to the calipers. Putting a manual master on a power system will result in ultra-sensitive brakes. The power brakes have the benefit of the booster to assist your input force, therefore less output pressure is required.

lb bore area psi
100 1 0.785 127.32
100 1.125 0.994 100.60

 

All this in mind...what's the best bore master to use in front of a Hydro assist unit? A power master would be better at avoiding sensitivity issues right?

 

I think the opposite is true: 1" bore requires less force to build the same static pressure than 1 1/8" bore. A 1" piston requires a longer travel to move the same volume of fluid a 1 1/8" piston moves with a given travel. A larger bore will provide more volume out for the same piston travel.

 

It's all about the hydraulic ratio between the caliper piston size and the master cylinder, the line has little to do with it and is the same for both MCs. It IS all about surface area. Since pressure = force/area if you have a fixed pedal ratio and you put the same amount of force on either MC the one with the smaller area (piston) will give a higher line pressure, however for fluid displacement it will take a longer travel to displace the same amount as the larger mc.

You are thinking in terms of a displacement pump forcing a volume flow through a restirction (principle of fuel pressure regulator), with a hydraulic system you are not so much pumping fluid, you are merely displacing it within the internal volume of the system through a transport line (the brake line in our case) and the ratio between the hydraulic cylidners at each end give you the hydraulic ratio and the ability to have a small cylinder move a much larger one and generate a large force over a large surface w/ a small travel at the penalty of having to move the smaller cylinder further to generate the fluid displacement to move the larger one, the small cilinder is able to create the high line pressure without much input effort.

So you go from low input force, large travel generating high line pressure to a small travel high output force.. the basics of a master/slave combo.

Apart from messing with the master bore size you can also slightly alter the pedal ratio, moving the rod up near the pedal pivot makes the pedal easier to depress at the loss of fluid displacement, requiring a longer pedal, move it down and effort goes up but pedal travel goes down.

 

Sorry guys. I stand corrected, again!

 

So, what do you think?

1. Will a new 9" dual diaphram booster have enough travel to push the piston of a new 1" bore master cylinder?
2. Will that combo be too sensitive for casual daily drives and commutes?

Confession time: I accidentally ordered a 1" bore mc and I am trying to decide whether to send the mc back when I get it or give it a try. :o

 

Send it back.
It probably also has a deep pushrod hole which would not work at all since the short pushrod wouldn't hit bottom.

Also with a 1" bore on power brakes they would feel really soft, I doubt you'd like it.

Keith