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Hot Rod Arithmetic Master Cylinder To Wheel Cylinder Bore Sizes, School Me

Discussion in 'The Hokey Ass Message Board' started by LesIsMore, May 25, 2013.

  1. LesIsMore
    Joined: Apr 8, 2008
    Posts: 449

    LesIsMore
    Member
    from Ohio

    Ok, so I have started other brake posts this week and have got a lot of good information as i troubleshoot my system problems. I have all new parts at this point and in some cases round 2 of new parts. My problem lies in the fact this car is retrofitted. I have 1946 Backing Plates, brakes and cylinders, added a dual master cylinder and residual valves as well as all new lines. I have done virtually everything under the sun to fix the half pedal on first pump, the best advice I have been given is my bore size in masted cylinder may not be enough on first stroke for the wheel cylinders.

    I have a Dorman Dual Master Cylinder 1" bore piece now for an early model Mustang, I had previously a Speedway unit, but ditched it because I have head they have issues and I have been trying everything.

    So the question is....If I have 1 1/4" bore wheel cylinders up front ant 1 1/8" in the back, what bore should my master cylinder be?

    If you know the proper bore, do you you which master cylinder (Car Year, Make and Model) will work? Parts houses generally need a year, make and model to help, since this is a retrofitted application for safety and to keep cool old components I am having a hell of a time.

    Please Help Brake Gurus....
     
  2. KoolKat-57
    Joined: Feb 22, 2010
    Posts: 3,027

    KoolKat-57
    Member
    from Dublin, OH

    To the best of my knowledge, you would need at least a 1 1/8" bore master cylinder. The 1 1/4" front wheel cylinders will need more volume, so even with all the brake shoes properly adjusted the pedal may have a slightly longer stroke before engaging the brakes. The 1 1/4'' wheel cylinders on the front will have a little more stopping power though. Not a bad thing!
    I would check Raybestos or Wagners web site for more info on master cylinder choices.
    Good Luck and be safe!
    KK
     
  3. Dane
    Joined: May 6, 2010
    Posts: 1,351

    Dane
    Member
    from Soquel, CA

    Can you be more specific? If your wheel cylinders don't have the modern type cups and spring you are probably feeling the result of the cups backing off. After the second pump of the pedal how long till the problem shows up again?
     
  4. The smaller the bore of the master the more braking power you will have
    BUT the further the pedal travel will be. If you apply 300 lbs force to a master of 1 inch dia piston then the actual pressure of application at the cylinder will be what the intial force times the ratio of the masters piston area to the slave cylinders area. I like a master bore about 7/8 to 1 inch for most applications. The residual check valve which is in all masters designed for Drum brakes pecludes the need to snug up the cylinders with an extra pump. Its purpose is to keep the pistons of the wheel cylinder tight against the pins but without enuf force to overcome the brake springs tension.

    Don
     

  5. LesIsMore
    Joined: Apr 8, 2008
    Posts: 449

    LesIsMore
    Member
    from Ohio

    Here's an informative article:

    Pedal Ratio
    The critical component in the braking equation is the pedal ratio. In operation, the brake pedal acts as a lever to increase the force the driver applies to the master cylinder. In turn, the master cylinder forces fluid to the disc brake caliper pistons or drum brake wheel cylinders. If you examine a brake pedal, you'll see the pivot point (where the pedal swivels) and the mounting point for the master cylinder pushrod are usually different. By varying the length of the pedal, and/or the distance between the pushrod mount and the pivot, you can change how much force (from your leg) is required to energize the master cylinder. This is the "mechanical advantage" or pedal ratio. This formula will help you figure it out: Input Force x Pedal Ratio √∑ Brake Piston Area = PSI.

    Mathematical babble? The arithmetic simply equates to the amount of force exerted by your leg times the pedal ratio divided by the area of the brake piston(s). FYI, the typical adult male can exert roughly 300 pounds of force (maximum) with one leg—and that’s a bunch. Something in the order of 1/3 or 1/2 that figure is obviously more comfortable, even in a hardcore racecar.

    The average manual (non-power boosted) master cylinder requires somewhere between 600-1,000 PSI to be totally effective. Somehow, 100-150 pounds of leg force has to be translated into 600-1,200 PSI. The way it's accomplished is by way of pedal ratio. While changing the overall length of the pedal is possible, it's often easier and far more practical to shorten the distance between the pivot point and the master cylinder pushrod mount location. That's precisely how many racecar chassis shops modify brake pedals.

    Brake Line Pressure
    Brake line pressure is a different thing than the force you apply to the pedal. Force acts in one direction and is addressed in pounds. Pressure acts in all directions against surrounding surfaces and is addressed in pounds per square inch or PSI. "Levers" (brake pedals) can be used to change the force. Inside the hydraulic system, the surface area of the piston is what is affected by pressure. Decreasing the bore size of the master cylinder increases the pressure it can build. Pistons in master cylinders are specified by bore size. But there's a hitch: The area of a circle (or bore) is Pi–R-Squared. The area of the piston surface increases or decreases as the square of the bore size or diameter. For example, the area of a common 1-1/8-inch master cylinder is approximately 0.994-inch. The area of an equally common 1.00-inch bore master cylinder is approximately 0.785-inch. Switching from the larger master cylinder to the smaller version will increase the line pressure approximately 26.5% assuming that pedal ratio hasn't changed.

    As the pedal force or the pedal ratio (or both) is increased, the stroke of the master cylinder is shortened (brake line pressure is unaffected). When the size of the master cylinder piston increases, the output pressure of the master cylinder decreases. A smaller master cylinder piston will exert more line pressure with the same amount of force (pedal ratio) than a master cylinder piston with a larger piston area. There's another catch: Since the brake line fluid pressure is working against the surface of the wheel cylinder (or disc brake piston), increasing the area of the cylinder will increase brake torque.

    The bottom line is, if the stopping power of a car needs improvement, or if there’s a need to reduce the pedal effort, several options are available: (1) Decrease the master cylinder bore size; (2) Increase the pedal ratio; (3) Increase the wheel cylinder bore size. If the pedal ratio is increased, there will be more travel at the master cylinder piston. If the master cylinder bore size is decreased, the piston has to travel further to move the same amount of fluid. Typically, a master cylinder has approximately 1-1/2-inch to 1-3/4-inch of stroke (travel). The idea here is coordinate the pedal ratio with the bore size to arrive at approximately half of the stroke (roughly 1-inch) in order to make the brakes feel comfortable, and of course, to bring the car to a grinding halt.
     
  6. LesIsMore
    Joined: Apr 8, 2008
    Posts: 449

    LesIsMore
    Member
    from Ohio

    And a Second

    Mathematical babble? The arithmetic simply equates to the amount of force exerted by your leg times the pedal ratio divided by the area of the brake piston(s). FYI, the typical adult male can exert roughly 300 pounds of force (maximum) with one leg—and that’s a bunch. Something in the order of 1/3 or 1/2 that figure is obviously more comfortable, even in a hardcore racecar.

    The average manual (non-power boosted) master cylinder requires somewhere between 600-1,000 PSI to be totally effective. Somehow, 100-150 pounds of leg force has to be translated into 600-1,200 PSI. The way it's accomplished is by way of pedal ratio. While changing the overall length of the pedal is possible, it's often easier and far more practical to shorten the distance between the pivot point and the master cylinder pushrod mount location. That's precisely how many racecar chassis shops modify brake pedals.

    Brake Line Pressure
    Brake line pressure is a different thing than the force you apply to the pedal. Force acts in one direction and is addressed in pounds. Pressure acts in all directions against surrounding surfaces and is addressed in pounds per square inch or PSI. "Levers" (brake pedals) can be used to change the force. Inside the hydraulic system, the surface area of the piston is what is affected by pressure. Decreasing the bore size of the master cylinder increases the pressure it can build. Pistons in master cylinders are specified by bore size. But there's a hitch: The area of a circle (or bore) is Pi–R-Squared. The area of the piston surface increases or decreases as the square of the bore size or diameter. For example, the area of a common 1-1/8-inch master cylinder is approximately 0.994-inch. The area of an equally common 1.00-inch bore master cylinder is approximately 0.785-inch. Switching from the larger master cylinder to the smaller version will increase the line pressure approximately 26.5% assuming that pedal ratio hasn't changed.

    As the pedal force or the pedal ratio (or both) is increased, the stroke of the master cylinder is shortened (brake line pressure is unaffected). When the size of the master cylinder piston increases, the output pressure of the master cylinder decreases. A smaller master cylinder piston will exert more line pressure with the same amount of force (pedal ratio) than a master cylinder piston with a larger piston area. There's another catch: Since the brake line fluid pressure is working against the surface of the wheel cylinder (or disc brake piston), increasing the area of the cylinder will increase brake torque.

    The bottom line is, if the stopping power of a car needs improvement, or if there’s a need to reduce the pedal effort, several options are available: (1) Decrease the master cylinder bore size; (2) Increase the pedal ratio; (3) Increase the wheel cylinder bore size. If the pedal ratio is increased, there will be more travel at the master cylinder piston. If the master cylinder bore size is decreased, the piston has to travel further to move the same amount of fluid. Typically, a master cylinder has approximately 1-1/2-inch to 1-3/4-inch of stroke (travel). The idea here is coordinate the pedal ratio with the bore size to arrive at approximately half of the stroke (roughly 1-inch) in order to make the brakes feel comfortable, and of course, to bring the car to a grinding halt.
     
  7. LesIsMore
    Joined: Apr 8, 2008
    Posts: 449

    LesIsMore
    Member
    from Ohio

    So, if all of the above is true, with my 1" bore MC and 1 1/4" front wheel cylinders, if I switch to a larger front wheel cylinder, say 1 3/8" should that help?
     
  8. Keep
    Joined: May 10, 2008
    Posts: 662

    Keep
    Member

    From my understanding. Discs need more pressure (smaller MC bore) Drums need more volume, i.e 1 1/8 bore.

    Adding a larger bore wheel cylinder will not help if the MC is not pushing enough fluid to fill it.
     
  9. seb fontana
    Joined: Sep 1, 2005
    Posts: 6,888

    seb fontana
    Member
    from ct

    ""As the pedal force or the pedal ratio (or both) is increased, the stroke of the master cylinder is shortened (brake line pressure is unaffected). ""

    Ah, no....
     
  10. Yes. A 1 3/8 wheel cylinder will give a good increase without requiring a whole lot more fluid (1.4848968sq in) over the 1 1/4 in cylinders(1.2271875 sq in). About 20% or a fifth.
    Better formula for area or easier is bore times bore X.7854. (Circle's area is 78.54% of a square of the same dimensions)
    Never gets confused with circumference (2piR) vrs area (piRsqrd)

    My trade school students used to say to me BTW "Pies arent square. They are round, sir.
    Don
     
    Last edited: May 26, 2013
  11. Fairlane Mike
    Joined: Sep 21, 2010
    Posts: 389

    Fairlane Mike
    Member

    Yeah, Don, same down here, except, "Cornbread R Square"!! ; )
     
  12. Fairlane Mike
    Joined: Sep 21, 2010
    Posts: 389

    Fairlane Mike
    Member

    In all seriousness, and BTW these things are a LEEEEEETLE hard to diagnose without seeing the "victim", ( car )! Question time; early Ford brakes; front AND rear? Did you plumb in a proportioning valve? The master cylinder you have is SPECIFICALLY for drum brakes front AND rear? Just for grins, what kind of car are you working on? I think you have a little glitch, because you should have good pedal IF everything is right! I've mixed up diverse brake components on several vehicles, that's why I think there's a little small detail screwing you up. Mike. BTW, how's the weather up there?
     
  13. LesIsMore
    Joined: Apr 8, 2008
    Posts: 449

    LesIsMore
    Member
    from Ohio

    Ok, so after some cross checking, and looking at specs in the catalog, I have found a 1 1/8" bore that has same specs I am looking for I think. 1971 C10 Chevy amongst others, so now all I have to do is find out if it has residual valves built in or not. Anybody know?

    http://www.summitracing.com/parts/dhb-m80568/applications
     
  14. JohnEvans
    Joined: Apr 13, 2008
    Posts: 4,883

    JohnEvans
    Member
    from Phoenix AZ

    Stop right here ! Pre 48 Ford brakes ALL used a single 1-1/16 bore master and everything worked fine with either the 1-1/4X1 or the 1-3/8X1 front cylinders. So your dual 1" MC should work just fine . The 2 pumps for a full pedal is a typical sign of miss adjusted brakes if they are otherwise solid. Are your shoes new and fitted to the drums? OS drums [+ .040 or more] that have not have had the shoes arced to fit will give simular results till the shoes wear in to give 80-90% contact. Using a larger than 1-1/8 MC will make for excessive pedal pressure to stop unless you now have a high ratio pedal setup, like on the order of 8-9 to 1 or more. That said what is the pedal ratio now? If more than 6-1 that also will translate to exessive pedal travel.
     
  15. LesIsMore
    Joined: Apr 8, 2008
    Posts: 449

    LesIsMore
    Member
    from Ohio

    Pedal ratio is where I run out of knowledge, I don't understand how to figure that out. There are so many variables here, I thought this would be an easy system. The more I read the more things that come up as potential issues, maybe I have the right components and shoes aren't broke in enough, or pedal ratio is wrong, or bore size is incorrect or it's adequate but I still have air in the lines...I think I am farther away from a solution than when I started. This is about the become a single master again, safe or not. I feel like I can't win.
     
  16. choppedtudor
    Joined: Nov 28, 2009
    Posts: 682

    choppedtudor
    Member

    while we're on the subject of brake system sizing...how does line size factor in? or not..
     
  17. LesIsMore
    Joined: Apr 8, 2008
    Posts: 449

    LesIsMore
    Member
    from Ohio

    I am running 1/4" lines.
     
  18. LesIsMore
    Joined: Apr 8, 2008
    Posts: 449

    LesIsMore
    Member
    from Ohio

    If brakes are misadjusted, why on the second pump is it so firm. If I continue to pump it stays firm, it's only when I let it return and rest for 8 seconds or so, I depress again ands its half pedal, second pump is hard again. My wheels currently don't even rotate a full turn, so I am not sure that it is it, but I am without a solution so trying everything. I bought a 1 1/8" mc today for a 1971 c10 pickup it is drum drum and I think (I guess) it has residual valves in it.
     
  19. Engine man
    Joined: Jan 30, 2011
    Posts: 3,476

    Engine man
    Member
    from Wisconsin

    I had never heard of drum brake master cylinders having residual valves until I joined this site. After seeing it in several posts I checked several master cylinders I had at home and others on vehicles at work and didn't find any. My test was to connect a brake line bent upward and pour fluid in it. The fluid drained into the master cylinder.

    Drum brake cylinders that I've seen have a spring between the cups that push the pistons out. I've often cursed these as sometimes they will push the boot off the end. Drum brakes have adjusters to keep the shoes close to the drum.
     
  20. JohnEvans
    Joined: Apr 13, 2008
    Posts: 4,883

    JohnEvans
    Member
    from Phoenix AZ

    OK more questions, were the brakes working OK before the MC switch? If so got to be either air in the system or defective MC I would think. Back in the days before self adjusting brakes in a car that needed the brakes adjusted you could get a higher pedal by pumping the brakes. What happened is if you pumped fast the shoes were not able to return all the way and you could get more fluid into the cylinders which gave you a higher pedal.
    Again are the shoes new and not been run or do they have some miles on them. As I said above if things worked OK with a single MC and thats the only thing you've changed is the MC that is where the issue is. How much clearence between the pushrod and MC piston? don't need much ,a 1/16 " is plenty.
     
  21. JohnEvans
    Joined: Apr 13, 2008
    Posts: 4,883

    JohnEvans
    Member
    from Phoenix AZ

    And by the way residual check valves do not do anything other than keep a slight pressure in the system to prevent air being sucked in around the piston cups on a fast release of pedal pressure. Most later cars [mid 60s up] had metal cup expanders that also helped prevent sucking in air.
    I've got over 50 years of screwing around with old cars and have only had 1 or 2 bad single MCs out of the box, must have had 8 or 10 bad duals out of the box in that time. And I do NOT work on cars for a living!!
     
  22. LesIsMore
    Joined: Apr 8, 2008
    Posts: 449

    LesIsMore
    Member
    from Ohio

    This is a fresh build, the brakes are all 42-48, with new wheel cylinders, and had a Speedway MC/Pedal universal Unit, I dumped it hoping it was junk, I bought a new Dorman 1" bore for Drum/Drum Specific system, upon hearing I may have too small a bore, I bought a 1 1/8" drum/drum MC for a 1971 Chevy C10 pickup, was redy to install, but have read hear about other variables leaving the door wide open for so many issues. I am lost here now.

    Currently I have a 1" bore MC in, still have a 10lb psi residual valve in front and rear system, new lines, and all new wheel cylinders, I am ready to change the MC again and swap the front wheel cylinders for 1 3/8" ones, but fear that wont solve it either. Not sure where to go next.
     
  23. JohnEvans
    Joined: Apr 13, 2008
    Posts: 4,883

    JohnEvans
    Member
    from Phoenix AZ

    Just for grins tighten up your shoe adjustment till you get a light drag ,no free spin at all and see what you get. New shoes will have small high spots that will give a false sense of adjustment . I always set a fresh set of shoes up with a light drag, acouple of miles and those spots will be gone and time maybe to re-adjust. I've noticed a lot of relined shoes the brake surface is not near as smooth as we used to get years ago.Kinda like the were hit with a 24 grit disk..
    Edit : Wait till you get things sorted out before changing the front WCs as those will need a bit more fluid to operate ,just what you don't need at the moment. The 1-3/8 bore cylinders were adopted to get a bit more brake on the front as the cars got heavier, probably don't need them on your build.
     
  24. vntgtrk
    Joined: Oct 25, 2012
    Posts: 6

    vntgtrk
    Member

    Don't mean to hijack, but I'm doing a '59 F100. Stock mc is 1 1/4. I'm using stock drums. I have a 1 1/16 bore mustang mc. Should it work? I also have an aftermarket 1 1/4 that I could use.
    My main question is with check valves. The totals aren't cumulative are they? I'm not sure of the 1 1/4" if it has check valves or not. I'm thinking of just adding some 10 psi valves to be sure of having 10psi front and rear
     
  25. mikes51
    Joined: Oct 4, 2001
    Posts: 2,195

    mikes51
    Member

    Agree with John, don't change your wheel cyl size. Someone suggested that to me, it will stop better. Well, the whole shoe, drum, etc. was not designed for that size. The brakes were too touchy, would practically lock up if I braked too hard. Also, the shoes wore out so fast I was down to the rivets and didn't know it. Besides anytime you are doing problem solving like this only change one thing at a time. Or else you won't know what caused what to change.
     
  26. JohnEvans
    Joined: Apr 13, 2008
    Posts: 4,883

    JohnEvans
    Member
    from Phoenix AZ

    vntgtrk: Just checked my brake parts catalog and your truck MC was orignaly 1-1/16" so the Mustang MC will be fine. 1 or 2 residials you will still have only 10lbs. That 1-1/4 MC would have been stock on a F-250.
     
  27. vntgtrk
    Joined: Oct 25, 2012
    Posts: 6

    vntgtrk
    Member

    Thanks a bunch
     

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