How do you calculate the bore of a master cylinder.

I think you are asking the volume of fluid that is moved in one full stroke of the brake pedal.
If that’s it then it’s Pi times radius squared times stroke.

If I misunderstood you please ask the question again

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Buddy
84 SE 2m8 (running but will get a carbed 2.8 from a rebody that is
getting an Archie V8 kit
85 GT (dead but will get the V8 from the 84)
http://www.geocities.com/buddycraigg/

Basically I'm looking to ensure I have the correct master cylinder by matching bore sizes.

Both the primary and secondary feature two different sized pistons, how do I find out if it is indeed a 1.125 bore?

Im not looking for the volume put out.

[This message has been edited by David Bartlett (edited 02-17-2003).]

Take it apart and measure it
or
the auto parts store you bought it from should have a specification book with bore size listed in it.

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Buddy
84 SE 2m8 (running but will get a carbed 2.8 from a rebody that is
getting an Archie V8 kit
85 GT (dead but will get the V8 from the 84)
http://www.geocities.com/buddycraigg/

I have it apart now. See pictures.

this is where i back pedal or side step the question completely since i didn't know that it had two different bore sizes.

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Buddy
84 SE 2m8 (running but will get a carbed 2.8 from a rebody that is
getting an Archie V8 kit
85 GT (dead but will get the V8 from the 84)
http://www.geocities.com/buddycraigg/

Don't average the bores!

The working bore is the smaller. The Quick Takeup bore is the larger. Each bore has a specific job.

When doing pressure math only the working bore is used. This bore is the one actually loading the calipers durring braking. This is the bore specified when only one is listed in the parts books.

The QT bore is only there to provide additional fluid volume durring the caliper takeup phase. This bore is often ignored in parts books. When ordering a whole MC, some books list it but most don't. It usually shows up in the listing for the seal kit used to rebuild a MC, but not always.

Also, you don't measure the piston. measure the actual bore. (Being extremely careful not to nick the bore.) The piston is always a bit smaller than the bore. The seal makes up the slight difference.

Read the brake service and upgrade articles in my cave. There is an explanation in one of them how the QT, AKA Step Bore, MC works.

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RIP Crew of Columbia. 01-Feb-03

Thanks Ogre. I've done more measuring and have come up with these values by measuring the actual bore.

* I got the picture out of your cave.

Anyways, this Master Cylinder is suppose to be one from a 94 Blazer, thus the 1.25 or 28.5mm bore. BUT from my measurements it seems to be too small being 24mm.

The quick takeup bore seems to be the same size as the Fieros comming in at 36mm.

I want to be sure that I bought what I paid for. What do you think Ogre?

i have to say two things,
one.
i am intersted in replacing my MC so i am watching this thread.
my brakes seem to work ok, but i would like the pedal to be a bit firmer.

two.
the name oger...
i am a tekken3 player (a video game that i became addicted to when i was repairing circuit boards for an arcade) and ogre is a chariter in the game so i picture his face when i read one of ogre's posts.

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Buddy
84 SE 2m8 (running but will get a carbed 2.8 from a rebody that is
getting an Archie V8 kit
85 GT (dead but will get the V8 from the 84) http://www.geocities.com/buddycraigg/

edited by Corona

[This message has been edited by buddycraigg (edited 02-18-2003).]

buddycraigg - if you keeping the stock calipers, it isn't a great idea to use a larger M.C. It will actually make your brakes worse. Check out the section in the Cave. I'm changing out all my calipers so I'll be needing a larger caliper volume.

i cant wait to read it...
i love discussing hydrostatic theory and application

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Buddy
84 SE 2m8 (running but will get a carbed 2.8 from a rebody that is
getting an Archie V8 kit
85 GT (dead but will get the V8 from the 84)
http://www.geocities.com/buddycraigg/

bump

David... If you measured 24 and the book says 28 then it sounds you've got the wrong MC. It's not uncommon to have the wrong one in a box ot for them to pull the wrong box completely. Many of these numbers are very close, even in sequence. It's not hard to grab the wrong one off the shelf.

buddycraigg Never use a larger MC with the OE brakes! As noted above that gives you weaker brakes. It Lowers the pressure in the whole system.

If you have low or soft pedal you have service problems that need addressing. Read the Brake Service article in my cave.

It could be bad parts or air in the system. It could be as simple as needing to manually adjust the rear calipers. Once you get the hang of manually adjusting them with the shim, it takes about 2 minutes a side after that. I recomend people check that every time the rear wheels are off. (They are supposed to self adjust. They often don't. Even when the hand brake is used.)

quote Originally posted by theogre: buddycraigg Never use a larger MC with the OE brakes! As noted above that gives you weaker brakes. It Lowers the pressure in the whole system.

(now I feel like I am arguing with my geometry teacher )
I want to increase the amount of pressure that is required to depress the brake pedal. Buy going to a larger bore master cylinder I will lose some mechanical advantage as the bore size ratios get closer to one another.

quote Originally posted by theogre: If you have low or soft pedal you have service problems that need addressing. Read the Brake Service article in my cave.

I have a 5000 pound car with manual 4 wheel drum brakes, “everything” feels like a soft pedal to me

quote Originally posted by theogre: It could be bad parts or air in the system. It could be as simple as needing to manually adjust the rear calipers. Once you get the hang of manually adjusting them with the shim, it takes about 2 minutes a side after that. I recomend people check that every time the rear wheels are off. (They are supposed to self adjust. They often don't. Even when the hand brake is used.)

I’ll have to check that out as I too agree that SELFadjusting rear brakes stands for “See Every Lift off Floor”

BuddyCraig, if you want the required force on the pedal to be greater for a given stopping distance, then you are correct; a larger M/C bore will give you less mechanical advantage and therefore require more force on the pedal.