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Brake master cyl: which bore where?

Discussion in 'The Hokey Ass Message Board' started by Gravehunter, Jun 12, 2010.

  1. Gravehunter
    Joined: Oct 3, 2009
    Posts: 24


    I have a Wilwood dual resivor master cylinder. No booster.
    One master cyl is 7/8" bore and the other is 1".

    Getting ready to hook up the lines today and i can't remember which master cyl is for the back and which is for the front brakes.

    Rear is a stock Ford 9" with drum brakes. Front is stock size Mustang II disc brakes.

    All i can find in searching is the larger capacity resivor goes to the disc brakes, but these resivors are the same size. So not sure if that means the 1" would go to the front or not?

    Speedway tech is closed today also.
  2. JohnEvans
    Joined: Apr 13, 2008
    Posts: 4,883

    from Phoenix AZ

    Larger bore = for the disk end as they require a larger fluid volume.
  3. Gravehunter
    Joined: Oct 3, 2009
    Posts: 24



    ok we are 1 and 1 here. I'm still confused! :D
  4. flamed34
    Joined: Dec 30, 2009
    Posts: 733


    It's a volume thing...1" bore for the discs. You've probably seen the "upgraded" bores of 1 1/8" to get even more volume. There may also be a larger line fitting on the larger bore MC.
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  5. Everyone is wrong, there is no constant or "stock" size among bodystyles.
  6. gasserjohn
    Joined: Nov 9, 2008
    Posts: 1,219


    THE FACT THAT THE DISC CLYS ARE LARGER IN RELATION TO THE MASTER CLY [&move less then drum clys have to move ect ect noones rite yet
    i know this is no help but my cars stop so????????????
  7. metal man
    Joined: Dec 4, 2005
    Posts: 2,948

    metal man

    I read this four times and have no idea what you are saying:D.Not to be the grammar police,but damn! Maybe it's just me getting tired. I would also like to hear some good,factual information on this subject.
  8. blue 49
    Joined: Dec 24, 2006
    Posts: 1,226

    blue 49
    from Iowa

    Volume is not really an issue since once the pads are seated, the shoes are adjusted up to the drums and the system is full of fluid, it's just a matter of whether the system is under pressure or not.
  9. Small MC Bore = less pedal pressure/more pedal travel. Large MC Bore = more pedal pressure/less pedal travel...everything else being equal. In "older" cars, the power brake option came with a larger diameter/less pedal travel MC...
  10. brigrat
    Joined: Nov 9, 2007
    Posts: 5,081

    from Wa.St.

    Are you sure it isn't;
    Small MC bore = more pressure/more travel
    Larger MC bore = less pressure/less travel
    We need an expert in the field!
  11. davidwilson
    Joined: Oct 8, 2008
    Posts: 595

    from Tennessee

    The bigger cylinder size goes to the disc brakes!!!!!!!!!!!!!!!!!!!
  12. RPM
    Joined: Feb 5, 2005
    Posts: 204


    This might help you out.

  13. Ford blue blood
    Joined: Jan 4, 2009
    Posts: 613

    Ford blue blood

    Disc brakes require almost no volumn to engage. The pads actually rest against the rotors (very lightly), pressure is what makes them work. Drum brakes require more volumn as the shoes are retracted by heavy springs to keep them off the drum. The new (since the late 40s) self energizing drum brake systems must have the shoes off the drum to prevent dragging. Having said all of that, master cylinder bore varies as does pedal ratio so modifying only one parameter will give restults of one kind. Smaller bore results in higher fluid pressure at the pads for a given amount of pedal pressure. Higher ratio (mormally 6 or 7 : 1) changes the amount of pressure at the pedal to give the same pressure at the pad. A 7:1 ratio will require less foot power to give 600 psi at the pads, a 6:1 ratio will require more foot power for the same pressure. As far as line hook up goes the rear port on the master goes to the rear brakes. It needs to "pump" up first to seal the front half of the master and allows the front brakes to energize. Ideal the rears engage first and lock up last for street driving.
  14. Smaller bore = higher pressure and less volume.

    The 1" bore should be for the disc brakes - they need the higher volume in most cases. Does your unit have the balance bar in it as well?

    Here is a fairly decent explanation of some of what you need to know - though it is for disc brakes. It will at least give you some ideas as to what this is all about and you can even use their calculator - based on your brake cylinders / disc sizes.
  15. 53sled
    Joined: Jul 5, 2005
    Posts: 5,819

    from KCMO

    Use a mustang II master cylinder.
  16. Okay - I still think some have it wrong . . . . I still believe the larger bore is for the discs, not the drums. The drums take less volume. If you're still pondering, then just wait until Monday and call tech support at Wilwood, they'll help you with everything and you'll not have to rethink whether you have it right or not (that is what I'd do!).

    Here are some more facts, these spell it out fairly well. Here is a link to the site - very good information:



    Your master cylinder is the heart of your braking system. In order for your system to function properly you must utilize the correct cylinder for your system's demands. The following brief descriptions should help you in determining what master you will need.

    A drum / drum master is designed to deliver fluid pressure and volume to the front and the rear of a braking system in equal proportions. Drum brakes will require less fluid and pressure than disc brakes. Typically a drum brake master will be smaller than a disc master and the fluid reservoir chambers will be equal in size. Since drum brakes require the use of residual pressure valves the original drum master cylinders had residual valves built into the outlets. Later model aftermarket units do not have these valves and they must be installed in the lines externally. Failure to incorporate residual valves will cause spongy brakes.

    Generally speaking it's not a good idea to use a drum brake master for disc brakes since the amount of fluid the cylinder pushes will not be sufficient.

    A disc / drum master is designed to push more fluid volume to the front disc brakes since disc brakes require more volume than drum brakes. Generally speaking a disc / drum master will have one fluid reservoir larger than the other.This is because the disc brake pads wear faster than the drum shoes and the fluid reservoir will drop faster. Original disc / drum masters had a built in residual pressure valve to the rear drum brakes only.

    Never reverse the outlets on a disc / drum master. Doing this will cause the front disc brakes to drag excessively from the residual pressure valve.

    A four wheel disc brake master cylinder is designed to supply more fluid pressure and volume to the rear disc brakes than the disc / drum master does. This is acheived through an internal piston re design. The piston that feeds the rear brakes on a disc / drum master will run out of stroke, limiting the amount of fluid pressure and volume that may be supplied to the rear isc brakes.

    The four wheel disc master redesign delivers the extra needed volume and pressure to the rear allowing your rear disc brakes to function properly. If you attempt to use a disc/drum master on a four wheel disc system you will get poor rear brake function and experience a spongy brake pedal with a long pedal travel.
  17. flamed34
    Joined: Dec 30, 2009
    Posts: 733


    Ok...I'll explain the volume thing like this, then bow out to let others argue.

    You have an input pressue which is calculated by the formula: P1=F1/A1 where P=pressure, F=Force, A=Area

    You then have an output force which is calculated by: F2=(A2/A1)F1

    So let's say you have 3/8" piston with an area of .588 sq in. And let's say that the action of pressing your brake pedal applies 50 lbs of force. Using the P1=F1/A1 formula, P1 would equal 50/.588 = 85 psi.
    Ok, so let’s do it with a 1” piston which would have an area of 1.57 sq. in. If nothing else changed, you’d end up with 50/1.57 = 31.8 psi
    WAIT! You say – That’s less PSI! Ah, here’s where more calculations are needed – specifically the one I mentioned above: F2=(A2/A1)F1
    F1 (Force 1) is the pressure you exert on the brake pedal (ignoring mechanical advantage here)
    F2 (Force 2) being the force the Caliper will apply to the rotor
    A1 being the area of the MC piston (we’ll use the 1” bore)
    A2 being the area of the Caliper piston

    A disc brake caliper has a piston in it of varying diameter - from, let's say 2" to 4" depending on the set-up/application. For our argument here, let’s use 3” as the piston diameter.
    Therefore, F2=(4.71/1.57)*50 = 150 lbs of force!!!!!!!!
    If we used the 3/8 bore, you’d end up with F2=(4.71/.588)*50 = 400 lbs of force.
    STOP – I just disproved myself! But wait, there’s a kicker!
    You also have to look at total travel (displacement) of the system involved, ie the caliper. Using the formula F1d1=F2d2 where d=distance and we’ll assume a MC stroke of 2”:
    Using the 1” bore:
    50 * 2” = 150 * d2
    100/150 = d2
    .66 inches of travel = d2
    Using the 3/8” bore
    50 * 2 = 400 *d2
    .25 inches of travel = d2
    F1D1=F2D2 – so 50 lbs of force over let’s say 2” of travel = 400 lbs of force over ONLY .25” of travel!
    So what am I trying to say? It’s a factor of pressure AND volume. You’ll never get the pressure you need if you don’t have the volume! You also have to calculate the mechanical braking surfaces in as well – a disc brake system positively engages a larger patch of the rotors than shoes engage a drum. I’d rather have 100 psi engaging 1000 square inches than 400 psi engaging 100 square inches.
    Wedge is 100% correct when stating that a smaller bore equals more psi. But you also have to take into account the total volume of the entire system, mechanical advantage at the pedal assembly, contact patch of the pads to rotors, etc, etc.
    Here’s a good page with a calculator (scroll down) showing the exact things I’m talking about. Enter dimensions in the fields and see the results!
  18. Gravehunter
    Joined: Oct 3, 2009
    Posts: 24


    Yes the Wilwood pedal/mc assembly has the balance bar built in it.
    Its a 6.25:1 pedal ratio.
    I'll go look through that site now.
  19. If nothing else, after you call Wilwood - update this thread on what THEY say, heck . . . it is their product!
  20. Gravehunter
    Joined: Oct 3, 2009
    Posts: 24


    Yep that basically it.
    Mine is the under the dash mount instead of floor mount but everything else is the same.

  21. Gravehunter
    Joined: Oct 3, 2009
    Posts: 24


    Yes i will.
    I got the pedal assembly about 3 months ago. I should have written it down but i didn't and now its just been so long i am not sure anymore.

    I think i'll just wait till Monday now and call them to be sure. Murphys Law almost never works in my favor. :D
  22. Gravehunter
    Joined: Oct 3, 2009
    Posts: 24


    I do want to thank everyone for chimming in with info etc..
    I knew there was alot of math, specs etc that went into sizing the bores but thought since i already had the bore sizes what one went where would be an obvious thing. But i guess not.

    I'll post back Monday on what Wilwood said.
  23. Giovanni
    Joined: Jan 21, 2010
    Posts: 173


    Remind me to never drive on the same road as you guys. Apparently 1/2 of you have bad brakes! ;-)
  24. drunkandgreasy
    Joined: May 20, 2010
    Posts: 100

    from nunya

    Oh damn thats funny.
  25. Now you have to pick which 1/2 you feel safe with :eek:
  26. flamed34
    Joined: Dec 30, 2009
    Posts: 733

    Member're right, I was using circumference formula. What I get for posting after no sleep on a turn around to 3rd shift
  27. Gravehunter
    Joined: Oct 3, 2009
    Posts: 24


    The verdict is: 7/8" to the front discs and 1" to the rear drums.

    I'll try to say exactly what they said here the best i can.

    The front pads ride against the rotors basically so they require very little fluid transfer(movement) to start to brake.
    The rear drums are farther away from the drums and require more movement to contact the drums.
    Thus a bigger bore to the rear than the front.

    Even though you can adjust the bias you also want more % of braking power to the front than rear of course. Since you use one pedal you want more pressure to the front to to aid in this. So the smaller bore gives more pressure to the front pads to aid in this.

    You could swap them and still be usable you would just have to adjust the bias bar(or proportioning valve if you use that instead) to compensate.

    Thanks for everyones input.
  28. Gravehunter
    Joined: Oct 3, 2009
    Posts: 24


    oh.. and as others have said here, they also said perssonal preference comes into play here but those are the general reccomendations.

    I also asked about that i've read and others have posted the discs needing more capacity. So one might assume the 1" went to the front.

    The capacity thing is that the front disc pads were alot more than drum pads. So as they wear the pads move into the disc more thus lowering the capacity level in the resevoir. So you need more resevoir capacity to compensate. That is just resevoir capacity but not bore size.
    Last edited: Jun 14, 2010
  29. ECIGUY
    Joined: Feb 24, 2009
    Posts: 111


    A little knowledge is a dangerous thing.
  30. chaos10meter
    Joined: Feb 21, 2007
    Posts: 2,191

    from PA.

    Pie are round, cornbread is square.

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